‘How many fish can I keep in my tank?’ is an age old question, asked by everyone who enters the hobby. Although the question is simple, the answer certainly isn’t. While there are guides out there, giving ‘number of fish per amount of water’ rules, these don’t take into consideration a number of factors.
When calculating volume of an aquarium, it is important to take into consideration the contents of the aquarium. Gravel/sand, rocks, plants and decor will all subtract from the overall volume of the aquarium. Therefore, when calculating volume, measure height from the top of the substrate.
Also, one litre occupies a 10cm x 10cm x 10cm area. This can be used as a rough guide for determining how much water certain objects inside an aquarium displace. For example, a piece of Malaysian driftwood that is 10 cm wide, 10cm thick and 10 cm tall will displace one litre of water. Obviously, you will never get a square piece of wood but you get the idea.
I personally round off the aquarium volume to the nearest 10 litres. Underestimating volume is generally better than overestimating.
The greatest flaw in the ‘fish per amount of water’ mindset, is that certain types of fish place greater biological loads on the aquarium than others. For example, a fully grown Oscar with place a far greater biological load on an aquarium than even a large school of small tetras. Large cichlids and catfish are among the greatest contributors to the biological load.
The food that you feed your fish also has a large impact upon the biological load placed on an aquarium. High quality flake foods and pellets place the least amount of biological load on an aquarium, while high protein foods place the greatest strain on the biological filtration. This is because proteins break down into nitrogenous wastes, such as ammonia. It is for this reason, that many people who keep Discus or other carnivorous cichlids feed their fish every second day; to minimise the strain placed on the biological filter.
Many aquarium stores maintain their aquariums at a pH of between 6.0 and 7.0. The reason for this is simple. In order to remain economical, aquarium stores need to place as many fish as possible into a single aquarium. This places a lot of strain on the filtration. The pH has a great effect on the efficiency of the biological filtration, as well as, the amount of ammonia in the water.
When the pH is below 6.0, the efficiency of the biological filtration is greatly compromised. However, the amount of ammonia in the water is also greatly reduced. Ammonia comes in two forms. Ammonia and ammonium. The lower the pH, the greater the amount of ammonia is in the ammonium form. Ammonium, unlike ammonia, is non-toxic. The opposite is also true, the higher the pH, the more ammonia is in ammonia, rather than ammonium, form. Therefore, aquarium stores have to find a happy medium between the efficiency of the biological filtration and the amount of ammonia in the water.
This is why many beginners are told to keep the pH as close to 7.0 as possible. A pH of 7.0 is probably the closest thing to a happy medium that you can get too.
So, how many fish can I keep in my tank?
There is no definitive answer. However, if you buy the biggest tank possible, don’t keep too many large carnivorous fish, keep feeding to a minimum and maintain a pH as close to 7.0 as possible, you could probably keep quite a few.