8 Common Beginner Fish Keeping Mistakes To Avoid

Last Updated: 7th November 2018

Fish keeping is an amazing hobby that can go a long way. It is a very rewarding hobby that will keep you deeply invested and the satisfaction will never end. But it is not as simple as just buying a fish tank and dropping your fish in it. There is so much to learn. But the learning experience is well worth the satisfaction in the end. The best way to learn is by doing, right? Well, I agree with this, but in case of taking care of your home aquarium, it is best to have the basics covered before you put your first fish in your very own aquarium. So I am going to share with you the most common fish keeping mistakes to avoid in the beginning phases of fish keeping. In this hobby, it is better to learn from the mistakes of other fish keeping enthusiasts. Do not be discouraged though. If you persist and keep at it, you will be very satisfied with this hobby. It is probably one of the most wonderful and low maintenance hobbies I have. No matter if you are just a beginner in fish keeping or have some experience already, you will find this article helpful. So these are the basic things you should know to prevent your fish from going ill, or even worse - dying. Here is an infographic about the top 5 common beginner fish keeping mistakes that you can freely share around, provided you link back to us as the source for this infographic:

Beginner fish keeping mistakes infographic

Buying the aquarium first and figuring out the rest later.

Gold fish in a small tank

If you do not have and aquarium yet or you have done this, then definitely stop right here and read this section. Many people make the mistake of ordering their aquarium first and doing their research later. This is very wrong. Whilst it is a must do thing, setting up an aquarium before introducing your new household friends, you should do your research first. Because if you buy your aquarium, you might have a very vague idea about what you want to put into it. But once you buy it, you are very limited to the options of what you can do with it. Except, of course, if you buy a huge home aquarium, then you can do pretty much everything you want with it. But if you are just starting out, most likely you will want to “test the waters” and see if this hobby is for you, right? So, most likely, you will go for a smaller aquarium. But once you do that, your options are limited.

If you know you want to keep a goldfish in your aquarium, then anything under 20 gallons won’t do. Yes, that is correct, keeping it in a bowl is completely wrong and is only harmful for your goldfish. And the myth - fish will fit the tank - is false. Each species of fish have their own requirements for their environment. Before buying an aquarium, figure out what is the best fish for you and what are their requirements. After you have found the fish of your dreams, find an aquarium that has adequate size, filtration and heating for its preferred environment.

Buying the tank and the fish at the same time. Not cycling it.

Empty fish tank

As already mentioned earlier, you must not buy the fish at the same time as the tank. This is a common mistake beginner aquarists make. You cannot just buy a tank, fill it with tap water and drop your fish right into it. Before you can place fish in your aquarium, you need to make sure that your aquarium has beneficial bacteria. This process is called cycling of the tank. The beneficial bacteria is needed to create the nitrogen cycle.

This probably is one of the most common reason for pet fish dying in aquariums. If someone says they bought a fish, put it in a tank and it died soon after, you can assume this was the cause for it. I’ll try to explain this as simply as possible. Fish leave waste after themselves which causes a buildup of chemical called ammonia in your fish aquarium. The beneficial bacteria inside the tank then consumes ammonia and creates nitrite. Then the bacteria consumes nitrite and produces nitrate, which in large amounts is lethal for fish. Plants and algae consume nitrate, so it is a good idea to have live plants in you aquarium for this reason as well. But it is not an obligation. What is an obligation, is partial water changes. Depending on your set-up, you need to change about 13 of your water from time to time. The frequency of this varies. Depends what kind fo set-up and fish you have. You should usually change the water in aquarium somewhere between 1-2 weeks time. Here is a simple video explaining the nitrogen cycle I just described:

So before putting fish into your aquarium, you should prepare your aquarium and let it develop for 2 to 3 days. You can get good bacteria by putting used filter sponge into your aquarium or a handful of populated gravel or substrate. After that you should monitor and measure your water parameters (and keep doing that every week or so). When the water is good, it is safe to let your fish swim and get acquainted with their new home.


Overfeeding fat fish

Now, if mistreated nitrogen is one of the most common reasons for pet fish dying, than overfeeding is number 1 for sure. This is a very common beginner fish keeping mistake, because the fish always seem hungry. And it so fun to watch them go after the food!

But the main reason why feeding your fish too much is bad, is because that, apart from damaging their health, creates excessive waste. And in the end, excessive waste builds into nitrate and will kill your fish. Also all uneaten food is going to stay in the tank. All the leftovers will start to decay and create cloudy water, algae bloom and affect pH and oxygen levels in your aquarium. One the factors to look at, when trying to figure out how much food to give to pet fish, is the temperature of the water. The higher the temperature, the more active your fish will be, therefore requiring more food than in low temperatures. A good rule of thumb is to feed your fish what they can eat in 2 minutes, without letting the food drop to the bottom of the aquarium. You should feed your fish about 2 or 3 times a day. I would say that 2 is enough.

Overcrowding the aquarium.

Overcrowded fish tank full of fish

This is related to the first mistake we discussed. If you have an aquarium, you cannot just put any fish into it and watch them go. You need to know what is the recommended aquarium size for your fish and how much waste they generate. In nature, you could not put so many fish together as you can right now thanks to the artificial filtration systems. Having too much fish in your aquarium will destabilize their environment and make it toxic. When looking for new inhabitants for your aquarium, you should find out what is the recommended number of fish per gallon. This is quite flexible metric more or less, but will give you a good idea about the number of fish you can stock in your tank. This ratio will also vary depending on the quality of your filtration system. If the fish you want is small in size and there is enough room, with high quality filtration system you will be able to sustain more fish.

Small aquariums are easier to maintain than large ones.

Small vs big aquarium fish tank

This is a myth. It is simply not true. The smaller the aquarium, the more delicate it is going to be. I think that for beginners, it is good to start with a small aquarium, because you will learn all the ins and outs of fish keeping. You will see what works and what doesn’t much better. But is easier to go for a bigger aquarium rather than a smaller one. This depends highly on your ambition height. If you know for sure you are going to to go into this with all your heart, go for a bigger aquarium from the very start. But if you are unsure, a smaller aquarium will be perfect.

So how come smaller aquariums are harder to maintain? Because it can hold smaller amounts of water. That means, things can go bad very fast. Because water in smaller space gets contaminated very quickly. With a bigger aquarium, you will have more time to prevent a catastrophe, if you check your water levels on a weekly basis. But if something goes wrong in smaller sized aquarium, it can be matter of days before your fish have become sick or dead. So you need to pay more attention to the status of water in smaller aquariums. Especially if you are a beginner in pet fish keeping.

Mixing incompatible fish

Fish fight incompatible fish

This can happen if you haven’t done any research about the fish you want to have. Fish are not so brain dead as many people think. They actually have personalities of their own. First thing you should find out, is how aggressive the fish is. Some fish, like male Bettas, are aggressive only towards other male bettas. Some, are aggressive against every other fish and cannot be put in aquarium that contains other fish. But most of the fish you will find are peaceful and friendly. They can coexist with others quite well. But do note their activity levels. While many of the fish are friendly, some may prefer more active lifestyle than others. And the more inactive fish that just want peace, will get stressed by , for example, the restless Endlers Guppies. Also some fish enjoy each others company. They swim around the tank in schools and may get stressed if they are left without companions. And stress usually leads to illness.

Not having plants.

Plants in aquarium

Okay, I actually don’t know how to describe this correctly. But the thing is, fish like to hide in spots where they can become invisible to everyone else. They use those spots for resting as well. This doesn’t mean that you need to have live plants for that. Just make sure you have enough interesting ornaments or fake plants for your fish to enjoy.

Although it is recommended to have live plants as well. Because they consume nitrate, creating a good environment for your fish and can be a source of nutritional food for your fish as well.

Obsessing with your fish.

Beautiful home aquarium

This is a tough one. Because from the moment your fish takes the first plunge into your aquarium, you won’t be able to take your of them. I guarantee this from experience. Fish, unlike the more popular pets, require minimal attention and care. The only thing they need, is being fed and living in healthy environment. Many beginner fish keepers tend to get too excited and start taking too much care of their fish. Do not clean the aquarium too often, as this will destroy the natural balance of it and also stress out the fish.

Another common mistake is rearranging the aquarium all the time. You must do this before you introduce your fish to it. Find that perfect combination you like and go for it. Prepare the tank before you introduce the fish. Because frequently rearranging things will only stress out the fish. So just sit back and enjoy the view!


As you can see, there is quite lot to learn. But do not be overwhelmed, this is no rocket science, just some basic things. If you have made it this far through our pet fish keeping tips, then you already know the basics and should be ready to take the plunge! You will learn all the details along the way, whilst enjoying your beautiful new friends. If you want to become a beginner aquarist, see if any of these beginner friendly fish is right one for you! Also, if you prefer to start with a smaller sized aquarium that can fit on any dekstop, be sure to check out our Best Small Aquarium Guide Aquarium can be such an amazing decor for any room, that you won’t be able to stop smiling about the many benefits it can bring to your household. So start your research and find the perfect set-up for you. And welcome to the great journey of pet fish keeping!