When looking at investigating the start of Piranha ownership, as well as picking the species you wish to keep and their individual requirements you will have to think about tank size.
So, how many Piranhas should you put in a 55 gallon tank? In short, a good number of Red-Bellied Piranha for a 55 gallon tank is 3. The black Piranha is a solo fish so only 1 should be placed in any tank, of any size. 3 Red-Bellied Piranha is the minimum number for a schooling fish so this is the minimum tank size with the minimum number of fish. It is also the maximum number of fish as Piranha like space, and lack of space produces stressed fish, which increases the risk of ‘fin nipping’.
Getting the tank size right Piranha is an essential part of Piranha ownership, perhaps more so than other fish, and there are also some nuances.
The pygocentrus Piranha, such as the famous Red-Bellied Piranha have a tendency to form schools, they do not like to be solo fish. Ironically, they are social animals.
By contrast, the famous black Piranha (serrasalmus rhombeus) is a solo fish. So the amount of black Piranha in any tank should be restricted to one.
There is a minimum tank size with Piranha, as they are not very small fish, and when they feel cramped they get stressed. With stressed Piranha they can often get combative and you will see increased fin nipping as they defend their space.
Normally the minimum tank size is around 60 gallons for Piranha, so 55 gallons is fine for the smaller Piranha, such as the Red-Bellied Piranha. In a tank they grow to around 10 inches in length.
The Minimum Number Red-Bellied Piranha For a Tank
The Red-Bellied Piranha is preferentially a schooling fish. They have a tendency towards collecting on groups for protection. They do this as a defense mechanism, not as a means of hunting via a pack.
The minimum recommended number of Red-Bellied Piranha for a tank is 3 to 4 fish, of the same age and size, with them having grown from juveniles together.
The best advice for Piranha aquariums is generally the bigger the better.
However, there are natural constraint from any owner, ranging from space to cost.
There then becomes a very real juxtaposition between the needs of space for a school of Piranha and the size constraints from the size of a tank.
It isn’t really recommended that an owner should go below 55 gallons for a Piranha tank, but if 3 or 4 Piranha are recommended for a 60 gallon tank, then for a 55 gallon tank, 3 Red-Bellied Piranha would be the best balance.
Placing 4 or 5 in a 55 gallon tank would be too many Piranha for the space, and as a schooling fish you can’t really serve their needs by only placing 1 or 2 in the tank. With only two placed in a tank, they tend to fight for dominance, rather than school.
Thus 3 Red-Bellied Piranha works best for a 55 gallon tank. For a black Piranha, as a solo fish, only 1 should be placed in the tank.
If you want to keep 4 Red-Bellied Piranha then you should really consider a 75 or 80 gallon tank as a minimum. They look big when you start, but Piranha grow very quickly in the first 10 months of life, and can reach 6 to 8 inches within half a year quite easily.
The General Tank Size Rule
Expanding the argument wider, as the rule for a Piranha is ‘the bigger the better’.
However, there is a general rule for schooling Piranha, like the Red-Bellied Piranha which is based on a 60 gallon tank minimum.
3 to 4 Red-Bellied Piranha should be in a 60 gallon tank and any other fish should be given an extra 20 gallons. Thus 7 Red Bellies will require a 140 gallon tank.
You can sometimes hear this expressed as the 20 gallons per fish rule. But 1 Piranha, like a Red-Bellied should never be placed in a 20 gallon tank.
When too many Piranhas are placed in an aquarium there tends to be a lack of space. With this lack of space, this stresses the fish. Fin nipping risks start arising and there is always the ever present threat that one day, one of the Piranhas will have been attacked and killed.
With the natural conflict of schooling Piranha and a limited tank size places a constraint on the choice of the amount of fish you can place in an aquarium.
You can’t have less than 3 Red-Bellied Piranha in a tank and each fish will require 20 gallons of water. Thus for a 55 gallon tank you really are limited to 3 fish only.
Of course that’s for a fish with a preference for schooling. The black Piranha, which doesn’t naturally school, only 1 can be placed in a tank, no matter what the size.