Nobody is perfect and we all make mistakes – particularly with our first goldfish tank and our very first fish.
This isn’t something to feel bad about. It’s normal for new goldfish keepers to make a lot of mistakes when they get their first goldfish tank or bowl.
Yes, ideally, everyone would research the correct way to care for goldfish and learn how to properly cycle a goldfish tank before getting their first fish. However, a lack of knowledge – combined with bad advice from some pet stores – often means this is not the case.
The important thing, of course, is that we learn and become better goldfish keepers over time. This means learning not just from our own mistakes, but from other people’s too.
That’s why we’ve put this list together – to help new goldfish keepers learn from other people’s mistakes.
Here, in no particular order, are the most common goldfish tank mistakes… and how to avoid them!
Don’t make these mistakes with your goldfish tank!
Mistake #1: Buying a goldfish bowl or very small goldfish tank
You should never keep goldfish in a bowl. Goldfish can grow to be over a foot long and need a lot of space to swim around. They also produce a lot of waste, which means that you need a large amount of water to dilute their waste and a large colony of bacteria to convert the waste into less harmful chemicals. All of these things require a big tank, not a small bowl. Read more…
Mistake #2: Putting lots of fish together in one goldfish tank
As mentioned above, goldfish need a lot of space and can pollute water quickly. You should therefore take care not to crowd your tank with too many fish.
Mistake #3: Not cycling your goldfish tank
Goldfish produce a lot of poisonous ammonia, which is very bad for them to swim around in. Fortunately, there are types of “good” bacteria, that can convert this ammonia into less harmful chemicals. Before you even get your fish, you should “cycle” your tank to build up a colony of this good bacteria. Read more…
Mistake #4: Feeding too much
Goldfish don’t have stomachs, which means that food passes through their bodies very quickly. All of this waste can pollute your goldfish’s tank water.
And as if that’s not enough, goldfish will continue eating as long as there is food available, without ever getting full-up. Because of this, you should be careful how often you feed your goldfish.
Too much food at once can cause constipation and swimming problems. Read more…
Mistake #5: Not changing your goldfish tank water often enough
Generally, you should change some of your goldfish’s water once per week. Use a test kit to keep an eye on your water parameters and do larger and more frequent water changes if you see spikes in ammonia or nitrite. Read more…
Mistake #6: Changing too much tank water at once
You should NOT change all of your goldfish’s water at once. We suggest regular water changes of around a third of the tank’s water. Use an aquarium siphon to vacuum your gravel and remove both water and waste from your goldfish tank. Read more…
Mistake #7: Not treating tap water
Water contains chemicals that can harm the colony of “good” bacteria in your tank. You should therefore use a product such as Seachem Prime to treat your tap water and remove dangerous chemicals before adding fresh water to your goldfish tank. Read more…
Mistake #8: Cleaning filters, gravel and decorations
Your filter sponges, gravel and decorations are where the good bacteria in your tank live. Never wash your filter, filter sponges, gravel or decorations in tap water! Instead, remove a bucket of tank water and use that water to gently clean them.
Mistake #9: Keeping incompatible fish in your goldfish tank
Make sure you only put suitable goldfish tank mates in your goldfish tank. Discover what fish can live with goldfish…
Mistake #10: Buying poor quality fish
Sometimes you can do everything right and care for your goldfish really well, but it will still live a short life. This can be due to buying “poor quality” fish, which are already weak, stunted or ill before you even bring them home from the pet store. Read our guide to buying goldfish to avoid this problem! Read more…
Mistake #11: Not quarantining new fish
New fish can sometimes carry diseases, which will spread to your other fish if you add them to your main tank straight away. For this reason, you should “quarantine” any new fish in a separate tank for two weeks, to make sure that they appear healthy and that no diseases appear.
Mistake #12: Not using an appropriate filter
As previously mentioned, goldfish produce a lot of waste, which can pollute their tank water. You should always use a filter in a goldfish tank in order to build up a colony of bacteria in your filter sponges and keep your tank water safe for your fish. Be sure to buy a filter that is large enough for your tank! Some filter series (such as the Fluval U Series) have various sizes of a particular filter design available. Read more…
Mistake #13: Not doing your research
Many new goldfish keepers buy (or win!) their first goldfish without doing any research first. They don’t learn about the 
environment needed for a goldfish to survive and thrive or how to look after goldfish properly. Of course, you’re on this website… so you’re already doing better research than a lot of goldfish keepers!
Mistake #14: Feeding the wrong foods or a diet that lacks variety
It’s not enough to simply pick up a tub of goldfish flakes and assume that all of your fish’s nutritional requirements are met! Like humans, goldfish are healthiest when they eat a varied diet. Check out our guide to goldfish food for more info.
Mistake #15: Cleaning the tank too much!
This may be surprising to some of you, but it’s definitely possible to be too thorough when cleaning your goldfish tank! For example, you should never wash your filter in anything other than water from the tank. This is because tap water will kill all of the good bacteria living inside the filter..
Read this article on cycling for more about these friendly little bugs!
Mistake #16: Not considering pH
Goldfish like a pH of around 7.4. However, it’s certainly possible to keep goldfish in water with a higher or lower pH than this. The most important thing is that your pH is stable. Goldfish don’t like it when the pH of their water varies wildly.
Mistake #17: Keeping goldfish in water that is too hot or too cold
Just like humans, goldfish are happiest within a certain temperature range. You should get a thermometer for your aquarium and ensure that your water temperature stays as steady as possible within the acceptable range.
Mistake #18: Not having enough aeration
A lack of aeration (not enough air in the water) is a particular problem in warmer water. The key thing to remember is that air enters the water at the surface. It’s not the number of bubbles in the water that matters, it’s how much the surface of the water ripples! More “surface agitation” (rippling!) from your filter and air pump, means more oxygen in the water.
Mistake #19: Adding dangerous ornaments
That castle might look fun and it may keep the kids happy… but it could be dangerous to your fish! Be careful not to add any ornaments that feature sharp edges or tight spaces. Your goldfish could become trapped in tight spaces and their scales and eyes can be damaged by sharp edges! This is particularly dangerous for bubble-eyed goldfish varieties.
Mistake #20: Adding gravel that is too small or too sharp
All goldfish love to stick their faces into the tank substrate looking for food. They also enjoy taking stones into their mouths and spitting them out again! Unfortunately, both of these activities can be dangerous if you have the wrong substrate. Be careful not to add sharp gravel that could damage your fish’s scales or eyes, or small stones that could cause your fish to choke.
Mistake #21: Adding too much substrate
You only need to add a thin layer of substrate (e.g. gravel or sand) to your tank. In fact, you don’t necessarily have to add that – it’s also possible to keep the floor of your tank completely bare.
The problem with adding too much substrate is that it’s difficult to clean and bits of food can get trapped between the pieces of gravel. This food then rots and pollutes your tank water.
Thanks. That was an excellent education.