30 Fishkeeping Mistakes Beginners Make – We’ve all done #18

Fishkeeping can be costly at the best of times without making any silly mistakes. Learning from the mistakes others have made can save you time, money and loss of life ( The fish, not yours) unless you’ve just done #28.

We took a poll on our Facebook fishkeeping group and asked over 2000 followers “What Fishkeeping mistakes beginners make” and from that poll we came up with the Top 30 Common Fishkeeping mistakes.

If you’re about to buy your first aquarium or you’re new to the hobby, then some of these mistakes may seem strange to you at first. But given time you’ll come to understand why so many people make the same mistakes over and over again.

Please don’t feel bad if you’ve already made one or even a couple of these mistakes. Remember, these common fishkeeping mistakes have been compiled from real life experiences by many fishkeepers from all levels of expertise from beginners to industry experts.

Here’s the original Facebook Post I posted:

This post was sent to several popular Fishkeeping Facebook groups to gather a true representation of fishkeeping mistakes made from all sectors within this hobby. All fishkeeping types from Saltwater, Freshwater and Coldwater fish keepers were asked the same question.

The hundreds of replies we received were placed in a spreadsheet and the Top 30 Fishkeeping Mistakes beginners make were selected and entered into this post.

fish keeping mistakes

We found from the huge response we received that there are many different mistakes that can be made but 30 stood out and were repeated across all areas of fishkeeping. There were a few oddball mistakes made which made us laugh but they were not common mistakes or repeated very often so didn’t enter our list. They were Funny, but were not made enough to get into our top 30! Like the example below.

 ” A man was filling a large quarantine tank in his basement with fresh water from a hose pipe and left it unattended whilst he went back upstairs to watch a TV program. He fell asleep and flooded the whole basement costing $3000 to dry and repair” 

That wasn’t one we heard very often not like the top 30 fishkeeping mistakes listed below have been made countless time. I think I have done at least half this list myself. lol

So let’s get stuck in!

The 30 most common fishkeeping mistakes made by beginners are:

  1. Impulse Purchases
  2. Using None aquarium safe silicone
  3. Cross contamination from QT tank
  4. Not securing Rockwork
  5. Not quarantining new fish
  6. Overfeeding
  7. Underestimating the running costs
  8. Not testing pH and Water quality
  9. Buying cheap and paying twice
  10. Using a Hydrometer instead of a more reliable refractometer
  11. Purchasing unsuitable fish for your setup
  12. Buying Picky Eaters
  13. Cheap food
  14. Not having an emergency plan for power failure
  15. Letting neighbors look after your fish whilst on vacation
  16. Adding too many fish too quickly
  17. Not getting the wife’s permission to buy a tank
  18. Being Impatient & Rushing 
  19. Not using RO-DI water 
  20. No lid on your aquarium
  21. Quitting too soon
  22. Accidental poisoning 
  23. Not doing enough water changes
  24. Overstocking
  25. Getting into fishkeeping
  26. Buying fish that will quickly outgrow your tank
  27. Tank too small
  28. Telling your partner how much stuff cost
  29. Incompatible fish
  30. Direct Sunlight

#30 Direct Sunlight

Placing your tank in direct sunlight or in front of your window may look good at first but over time the water will become cloudy and the tank will become green with microalgae. Direct sunlight can also raise the temperature of the aquarium and can fluctuate badly having a detrimental effect on your fish.

Direct sunlight won’t hurt your fish directly as they get direct sunlight in the wild after all, but it’s the change in water quality that will eventually harm them. Plus your tank will be covered in ugly green hair algae and no one wants that.

#29 Incompatible Fish

Mixing incompatible fish can cause all sorts of problems. Beginners make the mistake of buying fish they like the look of without asking for advice from the store. Before you know it they have a cute 2″ Oscar fish that will grow fast and will end up eating all their neons tetras.

Aquatic stores often employ very knowledgeable staff so make sure you tell them what fish you have and then ask them for their opinion which fish will suit the tank mates you already have and the size and type of fish tank you own.

#28 Telling your partner how much things cost

Fishkeeping is an expensive hobby and going out and buying a complete saltwater aquarium will all the equipment can cost thousands. So make sure you tell your partner before you go out and buy one or you may find yourself swimming with the fishes with concrete boots on. Haha

#27 Tank too small

Beginners fall into the trap of buying a fish tank that comes in a box ready to go with all the gear you need. Great! However, often you find that this type of fish tank around 15-20 Gallons soon become too small. The smaller the water volume the harder it is to keep and maintain good water quality. 

Also many fish soon outgrow your little tank within a year or two. You then end up paying for a second tank and starting all over again, a cost that could have been avoided if you’d only have purchased a nice size tank to start with.

#26 Buying fish that will outgrow your tank quickly

Many fish in the store is small, cute and inexpensive, however, they often grow to 12-15″ in size within a few years. Then what do you do with them? Many aquarium shops will not take them back when they are too big so you’re forced to increase the size of your aquarium.

Common fish that are purchased that outgrow their tank quickly are Plecostomus, Catfish, and Oscars. All are inexpensive and often imported at around 2″ and cost around $3-4. Tempting for any new fishkeeper, but don’t be. It can be a costly mistake to make buying one of those cute and tiny fish. 

#25 Getting into fishkeeping

We’re not sure about this one as we love the hobby but we know where you’re coming from. Basically, fishkeeping is very addictive and once you get the bug it can end up taking over your life, wallet, and home.

We would never put anyone off fishkeeping, after all, we love it. However, you may never have any spare cash ever again. We know hobbyist that miss out on holidays and new cars because they would rather buy a new tank, light or even an expensive fish. Don’t let this stop you but be warned it will take over your life. ( In a good way )

#24 Overstocking your fish tank

Overstocking can lead to many issues but the most common is high Nitrates & Phosphates. These will poison the water and can ultimately cause your fish to die.

There’s a formula aquarists use, 1 inch of fish per gallon of water. This is a good way to work out if your tank is currently overstocked but remember that most of your fish will still be growing and you need to factor this into the numbers.

Overstocking has many implications. It is vital to the health and well being of your fish that this issue is fixed as soon as possible. If you look at your tank now you may think they are fine, but it doesn’t take long for things to spiral out of control.

Poor water quality is the number one cause of illness and disease in fish. Toxins build up very quickly in an overstocked tank due too the amount of waste within the water volume. Partial water changes can help reduce the toxins but reducing the number of fish in your tank will be the best solution to ensure healthy and thriving fish.

#23 Not doing enough water changes

There are numerous reasons for changing aquarium water on a regular basis. The main reason is to keep your fish healthy. It’s often said that fishkeeping should be called ‘ water keeping’ because that’s what you do every day: look after your aquarium water. If the water in your aquarium is good your fish will be fine, as long as you remember to feed them!

During the Aquarium Nitrogen Cycle, Nitrates naturally build up in the water column. Regular aquarium water changes help remove these pollutants. During the water change, you will be removing harmful toxins such as nitrates, ammonia, nitrite, and phosphates. Not only are you removing harmful toxins but you’re adding essential minerals.

Water changes should be carried out every 2 weeks as a minimum and you should change 15-20% of your tanks water.

#22 Accidental poisoning

I have to confess I have done this one. I was very young and it was my first fish tank. A goldfish in a small  5 Gallon tank ( Not a bowl) you’ll be pleased to hear. I took my beloved goldfish out and placed him or her in a bucket and proceeded to clean the tank out with both boiling water and then washing up liquid.

The boiling water could have cracked the tank (which it didn’t) but certainly could have. After washing the tank out using a scrubbing brush and washing up liquid I felt pleased with myself and added new water and my beloved fish.

Minutes later I watch him or her swimming in a strange manner, doing loop after loop until it eventually died. Never use a chemical to wash out a fish tank as I did. Sorry, Goldy R.I.P

Do you know how long goldfish can live for? Read our article, you’ll be surprised! How long do goldfish live for?

We all learn and this is the reason for this post. To help other not to make the mistakes we have made. Other accidental poisonings can come from glass cleaners used on the outside glass which can accidentally get into the water. Metal contamination from nets, food clips etc.

#21 Quitting too soon

This often happens when people are waiting for the natural nitrogen cycle to happen. Beginners often get bored and give up before they really get to enjoy their fish tank.

Sometimes beginners, especially when setting up a saltwater aquarium give up for a number of reasons. The cost, the length of time for water qualities to settle, and the experience needed can often cause beginners to sell their tanks before they really get established and start to enjoy them.

#20 No lid on your aquarium

90% off fish especially saltwater fish are jumpers. Most fish will try to escape when first introduced to a new aquarium so it’s important to ensure you have a tight fitting lid on your aquarium. 

Reef tanks and nano aquariums look their best when viewed from above so I understand the lack of interest in covering your tank with a big ugly lid. You could try a mess/wire sheet on top of your tank, at least for the first few weeks after introducing a new fish. Often fish will settle down after a few days and not attempt to jump. This isn’t always the case but certainly will reduce the risk of losing a new fish.

There’s nothing worse than spending a few hundred dollars on a fish only to find it laying dead on the floor the next day.

It’s not just saltwater fish that jump, most freshwater fish will also try to escape if you have no lid fitted. Fish that are stressed will try to jump out of your tank and when first introducing fish to their new home is when the fish are the most stressed. Also if a fish is being bullied by another fish can cause them to jump.

Never overstock your fish tank

#19 Not using RO-DI water

Why is it important to use RO-DI water in a saltwater aquarium I hear you say? Simply put, you don’t know what is in tap water until you test it for yourself. Fluoride, Chlorine, Chloramines, Nitrates, Phosphates, and even metals like copper are in our tap water. All of which are harmful to aquarium fish. 

RO-DI means: Reverse Osmosis &/or De-Ionized water. RO-DI water is the purest water you can use in any aquarium, ensuring nothing unnatural is added to your aquarium. Even old copper pipes that lots of home have can be dangerous. Running your tap water through an RO-DI unit will remove unwanted elements. We like the LIQUGEN RO-DI Machine available from Amazon.

#18 Being Impatient & Rushing 

This was the number one commented on mistake that beginners make. It can be split into different categories but all involved rushing or being Impatient.

Examples were, adding fish to a saltwater tank before the full water cycle had occurred and the water was safe to add fish. The other was rushing and overstocking your freshwater tank with either too many fish or fish that were too large for your tank without thinking about the suitability.

Building the perfect tank with amazing water quality and great decoration with healthy fish is a time-consuming process. The best tanks happen naturally and take time! Don’t be tempted to rush and spoil all your hard work. Ask for advice and be patient. The results are worth waiting for.

#17 Not getting the wife’s permission to buy a tank

If we could stop you making just one mistake from this list of 30 fishkeeping mistakes beginners make then this one would be it.

Having the support of your wife or partner is important. Fishkeeping is expensive and often takes up all your spare time. This could be shopping for new fish or equipment at the weekends or performing partial water changes on your day off all of which can get you into trouble with the wife if you don’t have their complete approval.

Before purchasing a fish tank make sure you involve your wife and try to get her interested and looking forward to your new purchase. Above all…. Get their permission!

Involving the whole family is a wonderful thing, we love going shopping for new fish and our whole family including the wife really look forward to buying a new fish. The only problem is all of us agreeing on which fish to buy. LOL

#16 Adding too many fish too quickly

For many beginners, the temptation to add too many fish to an aquarium all at once can be overwhelming. But adding too many fish can destabilize the water chemistry of your aquarium and can cause the nitrogen cycle to fail.

As a good rule of thumb only add a maximum of 3 fish at a time to your tank. Once you’ve added new fish to your tank, wait until the chemistry of the water stabilizes again before adding more fish.

A good balance of fish, plants and a natural ecosystem will take time and adding too many fish at once will only lead to disaster. One of the pleasures of fishkeeping is going shopping for new fish, so why rush and buy them all at once? Enjoy stocking your aquarium slowly.

#15 Letting neighbors look after your fish whilst on vacation

I have made this mistake and it looks like others have to after reading some of the replies to my message on Facebook. Asking a none fishkeeping to look after your aquarium for anytime longer than a couple of days can lead to all sorts of issues.

The biggest mistake seems to be overfeeding or forgetting to feed them. They either forget to feed your fish which can be particularly bad for certain hard to feed saltwater fish, or they overfeed your fish and cause a catastrophic crash in your aquariums filtration system causing Nitrates and Phosphates to skyrocket which can wipe out a whole tank quickly.

If you’re going away for a long period of time then other factors come into play. Dealing with an outbreak of disease within a tank can be a big problem. They simply don’t know what to look out for and how to treat it when it does. The other is performing routine maintenance like water changes and filter cleaning etc.

We heard tales of people cleaning biological filters in boiling water wiping out all the good bacteria that keeps the tank and ecosystem balanced. Another was a neighbor cleaning the glass with chemical-based glass cleaning which accidentally entered the tank and killed all the fish within minutes.

#14 Not having an emergency plan for power failure

Having a backup plan is critical to cover any emergency. Most beginners don’t know what to do if they have a power outage or filtration failure. Having spare equipment is a good idea. You don’t have to spend a fortune on loads of equipment but having a simple air pump, cheap heater and secondhand external filter costing around $30 could be a lifesaver.

In the event of a filter failure, you can transfer your filtration media to your spare filter and continue to provide adequate water conditions whilst you repair to main equipment. Heater failure is common and even though heater have come a long way they still are prone to failure. We have written a Best aquarium heater guide which might help you find a more reliable heater to prevent failure.

You should also have a plan for other possible events that can wipe out your aquarium. The weather has recently played a dramatic role in fish keeping. Many parts of the country experienced a heat wave and struggled to keep their aquariums at the correct temperatures. Not usually the sort of issues we face in this hobby with all the technology we have to control situations like this. But this could be an issue we face more and more in years to come.

Our top tip, Be prepared for all occasions and all possible failures!

#13 Cheap food

Beginners often make the mistake of buying cheap, poor quality dried flake food for their fish. This not only offers very little essential vitamin, proteins, and other valuable supplements but offers them little in the way of variety.

Can you imagine eating just Lettuce for the rest of your life? No, Me neither, so why would you feed that to your fish every day. Boring!

Providing a suitable and varied diet is part of being a responsible fish keeper. We enter this hobby with the intention of keeping healthy fish and providing them the best possible environment and the food is a bit part of that.

Ensure you give your fish regular live foods such as brine Shrimps, Bloodworms and give them a selection of either frozen food or freeze-dried alternatives. There are hundreds of different foods available these days and costing very little which means you have no excuse to not give your fish a quality and varied diet.

   Best Fish Food Here!

#12 Buying Picky Eaters

Many beginners, especially in the saltwater category, make the mistake of buying beautiful looking fish without realizing how difficult they are to feed. This is one of the biggest killers of saltwater fish in our hobby.

Picky eaters like Mandarin Dragonets, Tanks, Pipefish and certain Anglefish can be difficult to acclimatize to their new homes which often results in them not eating and dying within a couple of weeks.

Not having the correct environment and setup can also result in your new fish not eating. As an example, the Green Mandarin Dragonet fish eat lots of copepods and adding these fish into a plain fish only tank without any live rocks or refugium with copepods living and breeding inside will result in them dying within weeks, even days.

Refugiums offer a natural source of foods for picky saltwater fish.

When purchasing a new fish make sure you ask lots of questions to the shopkeeper around their feeding habits. I often ask them to feed the fish in front of me so I can see what it eats and then buy that same food and take it away with me. This will give me the best chance of the fish accepting foods when it’s introduced to my aquarium.

#11 Purchasing unsuitable fish for your setup

This can be split into many different areas. The obvious one is saltwater fish and freshwater fish living in the correct water conditions, but many other factors must also be considered.

Are the fish you’re buying suitable for the size of tank you own? Are they compatible with the fish you already have?

Many factors must be taken into consideration before buying new fish. Ask the shopkeeper for advice, they are the best people to talk to so you don’t make any silly mistakes. It’s is very tempting to buy impulse buys when shopping for new fish. But you’ll regret it within hours if you get home and put the wrong type of fish in your tank!

Water qualities are one of the most important measurements and factors in keeping certain fish. Thankfully there are some amazingly accurate test kits on the market today to help you control the water qualities. We use SALTWATER MASTER TEST KITS available from Amazon.

#10 Using a Hydrometer instead of a refractometer

Using unreliable equipment can be a big mistake and an expensive one. Many beginners use a Hydrometer that you fill with salt water and watch the dial move to show you the salinity reading of your aquarium. Unfortunately, these are very unreliable and not very accurate resulting in a fault reading sometimes up to 5% difference. 

Using a Refractometer is a much more reliable method of reading the salinity of your tank. They are a little bit more expensive costing around $30 for a good one. We found this one on Amazon with good customer ratings. Professional automatic refractometer.

Spend the extra $15 or so and get an accurate reading for your saltwater tank. It’s money well spent.

#9 Buying cheap and paying twice

This one was mentioned in our survey a few times and seems to be a popular answer. When first starting it can be a big shock how expensive good equipment is. However, from past experience myself I know that buying cheap often means paying twice.

We have seen some great looking external canister filters online that claim to run 100 Gallon plus fish tanks for only $40-50. How good do you think these filters are? Not very is the answer. Most will break within a year meaning you’ll need to pack it back up and send it back to the online store. In the meantime, you’ll have to rush out and quickly buy another to ensure your tank continues to run.

Buying cheap is never a good idea. Have you heard to saying ” You get what you pay for?” This is very true.

Don’t be tempted to by cheap, unknown brands that could let you down. Buy equipment that is tried and tested and has received good quality customers reviews on Amazon or eBay.

#8 Not testing pH and Water quality

Most experienced aquarists check the quality of their water weekly but for beginners, they simply don’t know what to check for or how to do it.

One of the most common questions we get asked daily is ‘ why is my water cloudy” or ” Why are my fish dying” The first thing we ask about is the quality and results from their water tests and more often than not they reply ” I haven’t done any”

You need to know what’s going on inside your aquarium and conducting regular water tests is the only way you’ll know where your issues lie. Otherwise, you’re just guessing.

In a saltwater tank, you’ll need to test for more things than in a freshwater tank.


  • Nitrates
  • Phosphate
  • Dissolved oxygen
  • Nitrite
  • Ammonia
  • pH
  • Temperature
  • Specific gravity
  • Carbonate hardness
  • Calcium content


  • Temperature
  • pH
  • Ammonia
  • Nitrite
  • Hardness
  • Alkalinity 
  • Chlorine

Ensure you use a good quality water test kit and perform these test weekly to ensure if somethings going wrong you have the information to get t help quickly.

#7 Underestimating the running costs

Not only is buying and setting up a new aquarium expensive but the running costs can also add up. Electricity costs, salt, food, medicine, equipment, lighting all add up.

Good quality food for a large marine fish tank can cost $10 a week that’s if you use top quality dried foods mixed with live food and frozen treats. Chopped up mussels, prawns and other seafoods can cost even more.

Ensure you can afford the running cost before you buy and set up any fish tank. It’s hard to sell secondhand fish tanks and equipment if you decide to give up because it’s costing you a lot to run. Then you have the issue of giving your fish away. Most store will not accept your fish back if you can’t look after them.

The average cost of running a freshwater aquarium for a year is around $125 and for a saltwater aquarium rockets to over $300.

Running an aquarium can be very expensive

#6 Overfeeding

I think we have all been guilty of this one. It’s especially tempting for newbies and beginners to overfeed their tank. We often believe our fish are still hungry as they continue to come to the surface looking for food even after they have just been fed. 

However, overfeeding is one of the biggest problems in the aquatic world. If you overfeed your fish you must react quickly before you harm the water quality of your tank. Read our postWhat to do if you overfeed your fish it will help you avoid damaging your water quality and harming your fish.

Most fish would benefit from being fed 2-3 time a day, or smaller more regular portions. This means there will be less food left in your tank to become moldy and cause issues. This is the main reason why overfeeding is an issue. If there is more food than your fish can eat within 30-50 seconds it usually means some food will drop to the bottom of your tank or get sucked up into your filter where it will rot away or go moldy.

Remember, small and often portions work best.

#5 Not quarantining new fish

Most aquarium shops and pet shops are very good at quarantining their fish ensuring they sell only healthy fish. No matter where you buy a fish from or how healthy it seems you should always quarantine your fish to ensure you’re not introducing disease into your tank.

It is essential to quarantine new aquarium fish because they can carry parasites, most common are external parasites such as ich and velvet. … Quarantining new fish in a quarantine aquarium can prevent introducing disease into your tank.

The old rule springs to mind. ‘ Better safe than sorry’

#4 Not securing Rockwork

Not securing rockwork or using the wrong type of glue to hold rocks in place was another very popular mistake for beginners to make.

Creating a nice looking background and rockwork full of caves adds to the overall look of any aquarium but leaving the rocks stacked up without securing them can be a big and costly mistake. Many saltwater tanks but beautiful rock works built up from living rock consisting of caves where your fish can retreat to when they feel threatened. However, not securing them can lead to larger fish knocking them over and trapping your fish and hurting them.

We have even heard of rock work tumbling and cracking tanks. Using the correct type of sealant or glue is very important as you don’t want to accidentally poison your water. Glue and sealant with mold resistant additive are one of the worst ones you can use. We recommend just using approved aquarium silicone sealant use in the construction of all aquariums. This is 100% safe and strong.

#3 Cross contamination from QT tank

If you have listened to some of our previous advice and got yourself a separate quarantine tank to house your new or poorly fish, then great, well done. However, you need to be careful when looking after or doing maintenance on your QT tank.

Cross-contamination from this tank to your main one is very easily done. Simply using a net to catch the fish that was in QT and then transferring it to the new main aquarium is a simple way to spread disease. Also not washing your hands after working on the QT tank and then feeding your fish in the main tank can spread disease.

You have to be very careful with QT tanks especially if they house sick or diseased fish. Most people keep their QT tanks very close to their main tank, we suggest if at all possible to keep your QT tank well away from the main one and buy separate nets, buckets and food for this tank to reduce the risk of cross-contamination.

#2 Using None aquarium safe silicone

We touched briefly on this in #4 but it’s worth mentioning again. Repairing a small crack or securing decorations in place must be performed using the correct silicone. Don’t risk using anything like bathroom sealant or known cheap alternatives.

Go you your aquarium or pet store and buy some approved and well-tested glue or silicone. It’s not worth the risk to use anything that could potentially wipe out your tank for the sake of a couple of bucks. Silicones with mold resistant additives and ones that say they can be painted seem to be the worst culprits.

#1 Impulse Purchases

Okay, I admit it, I’ve done this one only very recently. I saw the most amazing Regal Tang and bought it without thinking. Only to add it to my tank with another large tang and they instantly started to fight. I left them for a day or two before having to remove the Regal Tang and tank it back to the store. Doh!

Make sure when you go shopping for new fish that you have done your homework. Go with an idea in mind and if you’re tempted to buy something out of the ordinary you ask the assistant a lot of questions. Ensure your tank mates will accept the new member of the tank and that you have the right setup for this new purchase.

Resist temptation!

Final Thought

We all make mistakes, that’s part of the hobby we love. We learn and grow very quickly in the fishkeeping world and especially if it’s a costly mistake. But don’t beat yourself up about it. Out of the 100’s of people who responded to my original question on Facebook ” What’s the biggest fishkeeping mistake beginners make” many of them where experienced aquarists who still to this day some 20 years into the hobby make silly mistakes.

The best thing you can do to avoid mistakes is read up about the hobby you’re entering into and ask ask ask. There’s no such thing as a silly question. There are so many wonderful and helpful people in the hobby who love to interact and help others. Join Facebook groups like ours Click here or join fish forums and post your questions online. Were a member of Fishforum.net and always post question and answers. Even after 30+ years in the hobby, I learn something new every day. 

We hope you learned something new today and hopefully, we have helped you avoid a mistake and helped you to get started in this wonderful hobby we love so much. 

Thank you for reading and please help us spread the word by sharing this post on social media. 

Happy Fishkeeping Forever!

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