13 Common Beginner Aquarium Mistakes You Should Avoid

Over the past couple of years, I have made a lot of mistakes. I am not going to hide the fact that I have probably made 90% of the mistakes in this article myself, but that obviously does not mean you have to make them as well!

I want to help you avoid some of the common beginner mistakes a lot of people make.

1 Not cycling tank

The first mistake is super common, and it is not properly cycling your tank before adding fish.

If you do not know what “cycling a tank” is, don’t worry. It means to let your tank run for about 6 weeks without any fish in it. During this period, a lot of bacteria will start to live in your filter. Sounds bad, but they are crucial to have before adding fish!

See, all fish will produce ammonia because they poop. Also, all uneaten food or organic plant matter will decompose and is also a major source of ammonia. And ammonia is super toxic to fish in low concentrations.

To get rid of this ammonia, the bacteria break it down first into nitrite and finally into nitrate. While nitrite is still toxic in low concentrations, nitrate is much less so. Therefore it is important to add your fish to a tank that already is completely cycled.

To know if you tank indeed is already fully cycled, read my article called “When is the nitrogen cycle complete in your new aquarium“, which will help you a lot I am sure.

2 Trying to get the pH to be perfect

This is a mistake that a lot of people make while they have the absolute best intentions. They know that the fish are keeping should be kept at a certain pH, for example 6.8. This is the absolute perfect pH for the fish, so they want to achieve this.

What they however do not know is that the same fish can live in a pH of 6.5 or 7.4 as well. What the fish needs way more than their perfect pH is a stable pH.

So instead of constantly trying to correct the pH by adding chemicals, catappa leaves or other ways to alter the pH, try and get the pH to a point close to the desired pH and make sure it remains stable over time.

The worst thing for a fish is a constantly fluctuating pH value.

3 Cleaning filter media with tap water


The goal of cleaning the media is just to remove some of the stuff that is stuck in between, not to make it look like new.

A lot of beginners clean their filter media by rinsing them under the tap. This can be lethal to all the beneficial bacteria that are living in your filter, and your tank will not be able to handle all the ammonia anymore.

This will kill your fish… so what should you do?

Instead of rinsing them under the tap, clean the filter media in a bucket of aquarium water. This way, you will not flush all the bacteria. Instead, hold it under water and squeeze a couple of times. You quickly will not be able to see your hand anymore due to everything coming out of the sponge.

After you squeezed it once or twice, you are good to go. Place it back and let it do its thing.

You should clean your filter once a month.

4 Overstocking your aquarium

Overstocking your aquarium is something that you should be very careful off. There are a lot of people who do this, and it bothers me. Not only should you provide all the fish with enough space to roam, it is also dangerous for them. It is possible that your filter is not able to handle all the waste that the fish produce.

To prevent this, make sure you tell the people at your local fish store how big your tank is and what other fish you already have in them. They will know what other fish you can put in or whether the tank already filled to the max capacity.

In my opinion, it is even more beautiful when there are less fish in your tank. It looks so much calmer, but that is just personal.

5 Overfeeding

This is a super common mistake, and one that I am also definitely guilty off…

Feeding your fish probably is the most fun part of keeping them. And if you have kids, they will want to feed the fish all the time because it is so much fun to watch.

However, when you feed your fish too much (more than they can eat) it will fall to the bottom of the aquarium. If you have bottom dwellers there is a big chance that they will find it, however maybe also not all of it.

All this uneaten food becomes a major source of ammonia because it will rot on the bottom.

An ammonia spike can be too much for your tank to handle, causing ammonia concentrations to rise. And even the smallest concentration of ammonia is toxic to your fish. It will definitely stress them, and potentially even kill them all.

Therefore, make sure you only feed the fish what they can eat. If you see that they ate it all, you can even feed some more and repeat.

6 Putting off water changes

Where feeding is the most fun part of keeping fish, doing water changes probably is the opposite. However they are super important to do regularly, especially in the beginning.

If you put off water changes and doing regular maintenance, it is possible that you will leave a lot of poop, uneaten food and dead plant matter scattered on the bottom. This is something that you normally would pick up when you do your regular maintenance.

As you know by now, this will turn into ammonia, killing your fish.

Even if you do not have a lot of junk in your tank you should take out, it is possible that by postponing water changes you allow the nitrate concentration to keep rising.

While we already concluded that nitrate is not toxic at low concentrations, there comes a point where even nitrate will kill your fish.

7 Keeping fish that do not fit together

I remember being super excited when I started looking at what fish I would be able to keep in my first tank. There were so many options.

And yes, I admit I wanted to keep them all. This is something that happens to a lot of us, and chances are this is a feeling you recognize.

But not all fish can be kept together! So always do your research before buying fish to put in your tank. Look online, ask your local fish store or look for other people that have experience in keeping those particular types of fish.

When I just got my baby turtles I wanted to also keep snails in that aquarium. Without thinking and doing research I put them in and…… one got eaten. That is when I realized they did not go well together as snails are near the top of the natural diet of the turtles. I saved the rest and put them in another tank.

8 Changing too much water at once

Doing water changes is crucial for the health of your fish. However there are a lot of people who do massive water changes. I am talking like 80%, 90% or even all the water in the tank. This will kill the beneficial bacteria in your tank.

Do not do water changes bigger than 50%, where 50% is already pushing it! For your regular water changes, change about 25% every other week.

If you measure ammonia in the water, or your need to do a big water change for some other reason, change 50% of the water. Nothing more.

9 Feeding bread to their fish

I have talked about this more on this website, but feeding bread to fish can kill them.

Bread contains yeast, which will expand when it comes into contact with water. This will cause serious constipation and might even kill your fish. It will bloat they bellies.

There are a lot of people who feed bread to their monster fish or to the fish in their pond. Just do not do this.

And yes, there are probably a lot of people out there who have been feeding bread for a long time and never have a fish die. That is just luck.

10 Buying the tank and the fish at the same time

Luckily a lot of decent fish stores will already not allow you to make this mistake, but chain stores or stores with less knowledgeable staff might allow you to buy the tank and your fish at the same time.

This basically comes back to mistake number 1, not letting your tank cycle. If you want to add the fish you just bought to an already cycled tank there is no way you will have the tank you bought with them already cycled right? As it takes about 6 weeks.

Instead figure out which fish you are going to keep first. Next find a tank that will fit their adult size and buy it. Let it cycle completely, and only when it it completely cycled you can go to the store and buy the fish.

11 Not regularly testing the aquarium water

Testing the water will give you so much insight in how your tank is doing.

It will give you a great overview of the exact values of all the water parameters. Using this knowledge, you can conclude whether your tank needs a water change, has old tank syndrome or needs other adjustments of some kind.

I think it is a mistake not to regularly test your water parameters at least once per month. Make sure you write them down so you can keep a proper overview.

Measuring the water values is especially important when you let your tank cycle.

12 Leaving the CO2 installation on at night

If you are keeping live plants, it can be a great idea to add extra CO2 to the tank to allow your plants to grow better.

A common mistake however, is to leave the CO2 installation on at night. This is a mistake because the plants do not need CO2 at night, because there is no light to do photosyntheses. Because of this, the CO2 concentration in the water will increase drastically.

With a higher CO2 concentration, the pH of the water will drop. This is dangerous to your fish, potentially killing them.

12.2 BONUS TIP Running an air stone together with adding CO2

Small bonus tip: do not run an air stone together with CO2 injection. The air bubbles will increase surface agitation. More surface agitation will remove the CO2 from the water at a higher rate. This way the CO2 will not reach the plants but leave the water almost immediately.

13 Adding too many fish at one time

Finally, adding too many fish at once definitely is a mistake that is commonly made by beginners. Especially when a new tank has just barely completed the nitrogen cycle.

If a tank is still new, the bacteria colony is not that strong yet. It can handle some ammonia, but not too much. If you fully stock your aquarium immediately, the ammonia will not all be broken down, causing the concentration to rise.

Instead, add only a couple of fish at the time. Slightly increasing the bio-load over time will make sure the nitrifying bacteria have the capacity to handle all the ammonia that is produced by your fish.

Bart Sprenkels

I have been keeping multiple aquariums since I was 18 years old. Just like many of you, I started with two goldfish but quickly learned they were not suitable for aquariums. Later, I switched to a tropical community tank and I also have two pet musk-turtles in a bigger aquarium. You can read more about me here.

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