Overstocked aquarium

aquarium by @shimp_father_uk

Wondering what the term 'overstocked fish tank' even means?

A richly decorated aquarium can look stunning. Just imagine lots of plants rich in colours, smaller fish and larger fish swimming around. All looks perfect, but unless you pay attention to how many fish you add, then problems may start appear. An overstocked fish tank is an aquarium with too many fish. We all like adding new fish after a trip to our local fish store but when we add too many fish, we reduce swimming room, fish don't have enough oxygen and all the fish will generate a huge amount of waste.

It is a common mistake to make for new fish keepers. You add fish and do not factor in the full adult size of each fish. This quickly results in fish not having enough room to swim. Bigger fish become often territorial and aggressive in too small aquarium.

Some big tank owners with very low stocking in their aquarium achieve results in keeping territorial species together in peace. They have of course more space to swim&relax and therefore meet less stress factors in a bigger tank.

How do you know your tank is overstocked?

Well, if you have noticed lethargic behaviour of some of your fish or perhaps few fish are drawn to water outlets. Are they taking a nap at the surface area? Also it is good to observe your fish gills, it can get brown and it can move rapidly . Those signs can be an alarm for you, that you may have too many fish and you have overstocked tank.

When you have too many fish in your tank, despite the obvious fact that your fish will have less swimming room, overstocked tanks end up with excess waste which will give off ammonia and nitrate which changes the water quality and water conditions.

Other reasons of ammonia and nitrate high levels in the fish tanks

Of course, overstocking is not the only problem of the higher levels of these toxic substances. This may be also overfeeding. Excess food is also the organic matter and therefore during the putrefactive processed it converts into these toxic substances.

How to know if your fish are overfed?

Firstly, if the amount of fish per liter of water (about) is correct, you have plants, you have the right filter, the water changes are big and regular etc. you can detect this problem simply by performing the aquarium tests. But one of the best indicators of overfeeding is just below :D

frozen fish food


Another indicator of too much feeding in the aquarium is the snail population. Pest snails when provided with the right amount of food have great conditions to reproduce. And the snails plague is not very welcome in our home aquariums.

How to fight the snails overpopulation?

Firstly, feed your fish in the way they can eat the whole portion in a minute or so. When snails stop eating this high protein fish food, the right conditions to reproduce reduce. But what to do with the already existing snails? See the solutions in our blog dedicated specially to this topic! I can tell you for now that the conclusion is that snails in the home aquarium are very good indicators of your fish diet! Many of them are also very beneficial for the aquarium ecosystem so let the small group be the diet good indication! ;)

What to do with overstocked aquarium?

Firstly, do not panic! Secondly safe your fish! Did you hear about nitrogen cycle? It is a chain, where decaying food and fish waste produce ammonia. They are converted to less harmful substances by nitrifying bacteria. However the right functioning of the balanced ecosystem depends on many factors. Therefore to have it under control start with measuring ammonia and nitrate in your fish tank. You can purchase measuring kit at your pet store (best water conditions: o ammonia, 0 nitrate). If your parameters are not too good, change around 50% water. The water change can be a kind of relief for your fish.

Restore some of your fish to new tank or container.

So that was your ASAP instructions.

However now you have to think how will you minimise your fish species in the near future to resolve problem with overstocked tank.

Basically, you have 3 options:

  1. Buy larger tank .

  2. Change to small tank, but buy few more of them.

  3. Find new home for your fish.

Number of plants

If there are a lot of greenery in the tank, they consume nitrogen compounds which means less nitrogen concentration. They also create valuable hiding places, but on the other hand, they take up space. When you have an overstocked tank, having enough space is important, and therefore, finding the balance between both your fish and your plants is crucial. Both fauna and flora also affect the water quality so the tank size is very important.

by @thh.earthlynonsense - IG profile


The better aquarium filter, the more fish you can keep in the tank (although this is not recommended). But we can't deny that an external filter to remove waste is much more efficient than an internal filter. They also take up less space inside, which means more swimming space. Instead of increasing your livestock with other large fish, it's worth keeping some specific fish species which can join the Cleaning Crew, e.g. Otos or black mollies. A few fish can support the filtration system of the tank efficiently. Having a nice cleaning-up crew is a true treasure for any aquarist because with its help, fewer algae problems are likely to happen.

As a summary, a general rules of the successful fish keeping are:

  1. Less animals in the fish tank, the better. The more fish in the tank, the worse ;)

  2. The better filtration , the better for your fish tank.

  3. Do not overfeed the fish in your fish tank.

  4. When buying fish from pet stores, keep in mind that they can grow.

  5. Monitor water changes.