4 red bellied piranha in a tank

How Many Piranhas In A 125 Gallon Tank?

Owning Piranha is a fascinating and engaging pastime. Most people choose to own the Red-Bellied Piranha, the most common of the genus and the most aggressive. As this fish is a schooling fish, selecting the right tank size is critically important.

So, how many Piranhas should you put in a 125 gallon tank? In short, the maximum number of Red Bellied Piranha would be 7. The Red-Bellied Piranha is a schooling fish so the minimum amount of fish in any tank would be 3, so the true answer for a 125 gallon tank would be between 3 and 7. With the non schooling species, like the black Piranha, you should only ever put 1 in any sized tank. With 3 Red-Bellies they will have a great deal more room than in 60 or 75 gallon tanks, so more decoration can be utilised which can reduce stress in the fish. There is therefore a reduced risk of fin nipping happening, or worse, a fatality within the tank environment.

3 red bellied piranha close up


With the acquisition of a 125 gallon tank to keep Piranha you are starting to double the recommended minimum size for the fish.

This is preferred as the Piranha can grow to be a good sized fish. Although they don’t quite grow as big in captivity, a big fish from the genus, like the black Piranha, even in captivity can easily reach 12 to 14 inches. The more commonly kep Red-Bellied Piranha can be around 10 inches in length, and as this species schools, a few fish will require a decent amount of room. The additional space, allows a handful of fish a much more stress free environment, which is critical to keeping healthy Piranha.

With too many Red-Bellied Piranha in a tank, relative to its size, increases the risk of stressing the fish. There is an increased risk of finding fish with their fins having been nipped. You may get fights for dominance, and there is also an increased risk of a fatality within the tank.

When considering the purchase of a tank, for any Piranha it is advisable to purchase the tank size they will grow into, not consistently have to change the tank size as your fish grow. When you place 5 or 6 juvenile Piranha in a 125 gallon tank they will look tiny, but the Piranha is a fast growing fish. 

They will grow at around 1 inch per month till they reach about 80% of their eventual size. Typically, for Red-Bellied Piranha they will grow to 8 inches within 6 to 8  months, and then grow another few inches over the next few years.

Rather than have to keep purchasing tanks, make sure your tank is right from the start.

6 red bellied piranha in a tank

The Number Red-Bellied Piranha For A 125 Gallon Tank

Unlike the black Piranha, which is a solo fish, meaning only 1 should ever be put in a tank, and the bigger the better, the Red-Bellied Piranha poses a more interesting question.

The pygocentrus genera of the Piranha are prone to schooling behavior, essentially they are a social fish, which offers a means of protection for each individual fish. It is an instinct for them.

Experts have determined that in the wild, Red-Bellies will form schools of around 20 and up, in order to form a protective block. Previously it was assumed that they formed groups in order to better attack large prey that were alive. It was determined that Piranha did not do this, they are rather shy and timid of perfectly healthy large animals. The schooling instinct is for defense.

Therefore, there should be a minimum number of Red-Bellied Piranha you place in any tank. It is normally 2 or 4 fish. They should not be placed in as individual fish or as  a pair. Ironically, 2 fish seem to have an increased risk of fighting for dominance and as a solo fish it will be more stressed, with no other fish present to protect it.

A very rough rule of thumb is that there should be 20 gallons of water per Piranha in a tank. Thus up to 4 Red-Bellied Piranha are suitable for a 75/80 gallon tank. For a 100 gallon tank, up to 5 or 6 Red-Bellies are acceptable.

The best advice is to always buy the biggest aquarium you can reasonably afford, the bigger the better as the saying goes. A bigger tank takes slightly more cleaning, needs more gravel and a bigger pump to produce light water flow but the theory is sound.

The tank should also be a normal rectangular shape. The fancy hexagon designs, are great for small fish but with Piranha it just limits their freedom of movement, a critical part of keeping healthy and stress free fish.

Thus, for a 125 gallon tank, up to 7 Red-Bellied Piranha sees a good compromise of space and amount of fish. Feel free to go lower, and only place 5 in there, that will greatly reduce the stress of the fish, but 7 is an acceptable maximum.

If you want to keep more Red-Bellied Piranha, say 8 to 10 fish, then you would need to increase the tank size to between 150 gallons to 200 gallons.

a very large piranha tank

The General Tank Size Rule

As previously mentioned, a good rule of thumb is to pick the biggest tank you can afford with the premise that the bigger the better, However, there are some limits on the practicality of that.

As an absolute minimum tank size, whether it’s for 1 black Piranha or 3 Red-Bellied Piranha, you should only consider sizes around 60 gallons and above. If you are going to only possess 3 Red-Bellies then 55 gallons might be OK, but the moment you go for 4, you need to think in the 60 to 80 gallon range.

While more Piranha might fit in the tank, you are really asking for more problems, with stressed fish in the tank. 

A good way to think of it is that a tank of 60 gallons will take 3 to 4 Red-Bellied Piranha and then you should add another 20 gallons per additional fish you wish to keep.

While searching around the internet you can find people who have 7 fish in and 80 gallon tank, they really are asking for stressed fish. With stressed Piranha, you will see ‘fin nipping’ as they clarify their dominance and territory, and one day there is almost certainly going to be a fatality.

Thus, if you want to keep a 10 Red-Bellied Piranha as a school, do so by all means, but give them the respect they deserve and make their habitat a stress free one, and purchase a tank 200 gallons plus.

What this could easily boil down to is simply remembering that every schooling piranha will need to given 20 gallons of space and you must have a minimum of 3 fish.

If they are stressed through lack of space, then as well as conflict within the tank you actually produce unhealthy Piranha, more prone to disease. You will find they need much less looking after if they have room to swim and hide, should they wish to.

underwater photo 3 piranha


So, a 125 gallon tank is a bigger tank than a recommended minimum, so you have a few more options than those choosing the smaller tank. 

You have the option of choosing up to say Red-Bellied Piranha or other schooling fish, or a singular black Piranha which, although will be a good size fish will have more room than those choosing a smaller tank. To reiterate, you should only ever have one black Piranha in the the tank. They are not a schooling fish.

Could you fit 10 Red-Bellied Piranha in 125 gallons? Yep, they will fit, but you are asking for problems. The fish will undoubtedly be stressed, and you should see a fatality at some point. Watch out for fin nipping as a sign that something is wrong if you want to do that.

If you care for this fish, then it’s a good idea to give them their recommended space. You might be lucky, but you are asking for more stressed fish, thus a higher chance of aggressive fish towards each other and more prone to disease.

As well as the tank size and correctly pairing that with an appropriate amount of fish, it’s essential to get the low level lighting right, the tank location where they will not be incessantly disturbed and give them good clean water along with a healthy diet, that varies from fruit and nuts to frozen fish and worms.

After all that, you can call yourself a successful Piranha keeper.


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