This is one of the most common questions when starting or rebuilding an aquarium.
And it's easy to see why. You want the aquarium to feel alive. The fish are often the heart of the tank, and an aquarium with too little life can feel a bit empty.
At the same time, you don't want to end up in a situation where the water is difficult to keep clean or where the fish can't fit in as they grow.
The best approach is therefore quite simple: start reasonably, follow how the aquarium is doing and adjust if necessary.
It does not have to be perfect from day one.
There is no exact figure
You may have heard rules like “one centimetre of fish per litre of water”.
It may give you a rough idea of how much to fill the aquarium, but it is not a very good rule to follow blindly.
A small schooling fish and a powerful moth do not burden the aquarium in the same way. A calm fish and a very active fish don't need the same space either. And the adult size of the fish is more important than the size it is when you buy it.
So instead of chasing a precise figure, it is better to think in balance.
A good aquarium has a margin
A balanced aquarium has little margin.
This means that the filter has time to deal with waste, there is enough oxygen, the fish have room to move around, and a missed water change is not an immediate disaster.
This doesn't mean that the aquarium has to be sparse or boring. It just means not pushing it to the limit.
Many fine aquariums feel alive precisely because the fish have room to behave naturally. A school can swim together. Bottom-dwelling fish can search around in peace and quiet. Fish that want protection can withdraw.
Look at the adult size
The most important mistake to avoid is just looking at how the fish looks in the shop.
Many fish are sold young and small. Some don't get much bigger, but others can grow quite a lot. This is the case for some catfish, goldfish and cichlids.
Before buying a new species, it is wise to quickly check:
- how big the fish will be as an adult
- if it wants to go into shoals
- if it needs a lot of swimming space
- if it becomes territorial
- if it fits with the fish you already have
It doesn't have to be a big research project. But these questions will help you avoid the most common missteps.
You have to try it out
The aquarium hobby is practical. You learn a lot by seeing how the aquarium actually works at home.
It is perfectly normal to adjust fish stocks over time. Sometimes you realise that a school could have been a bit bigger. Sometimes you realise that a fish is growing faster than expected. Sometimes two species don't fit together as well as you thought.
This does not mean that you have failed.
If a fish gets too big, or if the aquarium feels too full, there is often a good solution. Many aquarists donate or resell fish through local Facebook groups, aquarium societies, forums or contacts with others who have larger aquariums.
The important thing is to act when you realise that something is not working, not to have calculated perfectly from the start.
Add fish at a leisurely pace
An easy way to succeed is not to buy everything at once.
Start with a small group of fish. Let the aquarium run for a while. See how the water behaves, how the filter copes with the load and how the fish use the aquarium.
Then you can add more if everything looks stable.
This makes the hobby much less stressful. You don't have to decide on the final image right away. The aquarium is left to grow.
Water values are your security
The best way to know if the aquarium can cope with its fish population is to look at the water values.
Ammonia and nitrite should be at zero. If they are, and the fish are behaving normally, you are on a good footing.
Nitrate can rise over time and is kept down with water changes, plants and a reasonable load. If you notice that nitrate quickly becomes high between water changes, it may be a sign that the aquarium is quite heavily loaded.
But you don't need to measure everything all the time. In the beginning, after new fish or if something feels wrong, tests are particularly useful. Once the aquarium is stable, you can often learn how it usually behaves.
Furnishings help fish thrive
Roots, plants and rocks do not affect how much waste the fish produce, but they do affect how safe the aquarium feels.
An open aquarium without hiding places can cause some fish to become stressed, even if the actual number of fish is low. Roots, plants and shaded areas give fish more ways to move around and rest.
For schooling fish, a darker background or root environment can help them stick together. For bottom dwellers, roots and plants can create calmer areas. For territorial fish, the decor can break sight lines so they don't see each other all the time.
It won't make the aquarium biologically bigger, but it can make it much better to live in.
A simple rule of thumb
If you want a simple way of thinking, you can use this scheme:
- Choose fish that are suitable for the size of the aquarium when they are adults.
- Start with fewer fish than you think the aquarium can handle.
- Add more in stages.
- Look at fish behaviour and water values.
- Adjust if something is not working.
It goes a long way.
In short
You don't need to know exactly how much fish you can have from the start.
Try to choose species that are suitable for the size of the aquarium, build up the fish population at a slow pace and monitor how the aquarium is doing. If things go wrong, they can often be corrected, especially by giving away or selling on fish that get too big or don't fit.
The goal is not to be perfect. The goal is an aquarium that feels alive, stable and pleasant for both you and the fish.
Sources and further reading
The RSPCA has a good review of aquarium sizing, filtration, water changes and why fish size guidelines should only be seen as guidelines: Choosing an aquarium.
The RSPCA also writes about what to consider when keeping several fish together: Keeping pet fish together.