If you are looking to start owning Piranha the most common species to own is the Red-Bellied Piranha, also thought to be the most aggressive. As a schooling fish, tank size is important.

So, how many Piranhas should you put in a 75 gallon tank? In short, a respectable and healthy amount of Red-Bellied Piranha for a 75 gallon tank would be 3 or 4. Three Red Bellies is the minimum anyway as they are a schooling fish, but the tank is now big enough for you to think about adding another fish. With non schooling fish, like the black Piranha only 1 is still advised. With 3 fish the Piranha will have more room than a 60 gallon tank allowing for more decoration and space that will reduce stress in the fish. There is less likelihood of fin nipping or fights for dominance. 4 Red Bellies increases that risk slightly but should be OK in a tank of this size.

Introduction

With the minimum size of a tank for a Piranha around the 60 gallon mark, getting the correct size of aquarium is a critical part of Piranha ownership.

The reason is that the big fish, like the black Piranha can grow to an impressive 12 to 14 inches in length, and the much more common Red-Bellied Piranha forms schools, so requires a reasonably big tank to give the fish a stress free environment. Red-Bellied Piranha shouldn’t be put in to a tank either solo or just 2 fish. The results are invariably bad.

This minimum tank size helps the Piranha and its traits grow healthy and stress free. If you wish to keep black Piranha, the minimum tank size should be 60 gallons, but no matter what size of aquarium you choose, there should only be one black Piranha in it.

If there are too many fish in the tank, this increases stress and you may well get fights for dominance and as a result, one of the fish may sustain a fatal injury, or at the very least get the fins nipped with bites.

The size of the tank should be bought from the very beginning, even if you buy juvenile Piranha. The tank will look huge but piranha grow very quickly, often putting on 1 inch per month for the first 8 to 10 months before slowing down. That means within 10 months it is entirely possible with good clean water and a good feeding regimen you will have 10 inch fish in there.

The Number Red-Bellied Piranha For A 75 Gallon Tank

Being part of the pygocentrus genera, this fish will have a tendency to school. They do this more for protection than as a predatory act. All fish who are part of the school will act as a warning for others if a predator is nearby.

For this reason, the number of Red-Bellied Piranha is always meant to be around 3 to 4 fish, no matter what the size of the aquarium. However, each fish should be given approximately 20 gallons of its own space.

Thus, up to 4 Red-Bellied Piranha are thought to be good for a 75 gallon tank. 3 fish would give them more space which reduces stress, but 4 should be OK, 5 Red Bellies would require at least 100 gallons as a tank size.

In general, buy the biggest tank you can reasonably afford, as a good rule of thumb is the bigger the better. Within limits of constraint, a bigger tank requires bigger pumps, requires more cleaning and there may be limited locations in your home to place it.

Red-Bellied Piranha should never be located in small tanks or fancy designs. A simple, normal rectangular tank is the best choice. Too small causes a great deal of problems.

So, 4 Red-Bellied Piranha works best as the maximum number of fish for this size, but feel free to only put 3 in. If only 2 are placed in the tank they can fight for dominance and you should never place a solo fish in a tank if it is preferentially a schooling fish. 

Thus, for a 75 gallon tank you are left with the choice of 3 or 4.

The non schooling genus of Piranha, such as the black Piranha, being a solo fish, you should never place more than 1 in a tank. If a 2nd is placed in there, there is a higher likelihood of fatal attacks.

If you require more than 4 Red-Bellied Piranha then you should move up to a 100 gallon tank.

The General Tank Size Rule

If you are still deciding the right balance, the rule for a fish with teeth is ‘the bigger the better’.

With Piranha, only a tank size above 60 gallons should be considered. This really is the minimum, or thereabouts anyway. 55 gallons might be fine, but search the internet for long enough and you will see people with 5 fish in a 40 gallon tank. 

They will fit, but you are asking for a whole bunch of problems.

Whilst you can fit 3 or 4 Red-Bellied Piranha in a 60 gallon tank, any other additional fish should given another 20 gallons. If you want to own a school of 10 Red-Bellied Piranha you will need a tank size of 200 gallons.

Essentially if you think of it as the 20 gallons of water per fish rule to a minimum of 3 fish then you will have a good idea of what tank size you may require.

The danger is that Piranha need their space. If too many are placed in a confined area, the fish become stressed, and amongst other things prone to disease. Additionally stressed fish are more prone to attacking each other, thus a fatality may occur.

Fin nipping may be a sign that fish are stressed and require a bit more room.

Conclusion

While a 75 gallon tank is above the recommended minimum for Piranha, the need for Piranha to have their own space makes 3 or 4 Red-Bellied Piranha a good fit for this size of tank.

Could you fit 5 in? Sure. But there is an increasing risk of stressed fish. You may be lucky, but if you care for your fish, you should try to keep them healthy, both mentally and physically.

You certainly can’t have less than 3 Red-Bellied Piranha as it is a schooling fish of the pygocentrus genus, so is not meant to be kept alone. The black Piranha can, and indeed should be kept alone in a 75 gallon tank.

As well as getting the tank size right, make sure there is low level lighting, plenty of decoration for them to hide out of sight and feed them a varied diet in good clean water and you are well on your way to keeping a good healthy Piranha.