When Piranha hatch or they are bought from a pet store they will normally be under an inch long. In a 60 gallon tank this can look spacious… but wait, they will grow.

So, how fast will Piranha grow? In short, once they hatch, and assuming good conditions a Red-Bellied Piranha will have a growth spurt for the first 6 months and then slow down. They will grow at around an inch per month until they reach around 7 inches when they will grow at around an inch per year. In an aquarium, they typically do not grow to the size they would in the wild, and an adult Red-Belly in an aquarium will be around 9 inches and weighs about 5.5 lbs. In the wild a more typical adult would be 12 to 14 inches in length.

Brief Overview Of How They Hatch?

During breeding season, a male and female Piranha will pair and find some foliage. The male may dig a little nest in the water bottom and the female will lay a clutch or up to 5000 eggs. The male then fertilizes them.

The male and female will then guard the nest, and are fiercely territorial during this phase. It can last 2 or 3 days until they hatch. The warmer the water, the faster the hatching procedure.

Once they hatch, they begin to consume food and start to grow.

What Size Do They Grow To In Captivity?

As well as considering the speed at which Piranha grow, it’s worth noting what size you can expect from them.

In the lush rainforests of the Amazon basin, as a species the Red-Bellied Piranha can get to 50 cm (20 inches) in length and weigh up to 4 kg (8.8 lb). That is quite a decent sized fish.

However, an average size and weight is more realistically likely to be 30 to 35 cm (12 to 14 inches)

In captivity, they grow to a slightly smaller size. If you have a really huge aquarium they will grow slightly bigger, but a typical size for a Red-Bellied Piranha in captivity

In captivity you should expect to see them around 3 inches less in size. A Red-Bellied Piranha will typically be around 9 inches when it is fully grown.

How Fast Do Baby Piranha Grow?

This is pretty broad and depends on stress levels, water quality / temperature, and food quality. If you follow the rules of a spacious tank, good quality water and a varied natural diet roughly you could expect 1 to 2 inches per month until they reach around 7 to 8 inches and then growth slows down, when it might be around 1 to 2 inches per year.

Red-Bellied Piranha have a growth spurt early on to get to a decent size and then slow down. Typically you might see growth for 6 to 8 months until they slow down and grow at a much slower rate.

You can probably see why experienced Piranha keepers say you should buy the tank you want them to grow in to now.

How Long Until Fully Grown?

From fry your Red-Bellied Piranha should be around 6 to 8 inches in length, depending upon the room in the aquarium, until they really start to slow down.

An aquarium Red-Bellied Piranha will typically get to 7 inches and then add another 2 inches over the next two years, so after 2 to 3 years they will be fully grown, but they become adults quite quickly, and about 80% of their eventual size after 9 months.

Although these are rough figures, it is a guide to help you decide. There are many factors in play here including water temperature, water quality, aquarium room, stress, foliage, decoration and lighting. As well as a healthy balanced diet.

Also don’t forget the importance of exercise, a mild current can provide Piranha with the healthy regimen it needs.

They should max out at around 10 to 12 inches in length in optimal conditions (ie as close to the wild as you can get, including tank space).

How Fast Do Black Piranha Grow?

Firstly, black Piranha’s grow much bigger than Red-Bellied Piranha and are a solo fish. A 75 gallon tank for a single black Piranha isn’t uncommon. They need their space and typically take a while to get used to an aquarium. It also isn’t advised to put other fish in with them.

Black Piranha grow slower than Red-Bellied Piranha but grow bigger. It’s more like 0.5 to 1 inch per month and then 1 inch per year after they reach 6 inches in length.

Conclusion

So, species and healthy conditions matter to the growth of Piranha. While tolerant there are good replicate conditions you can provide to aid their growth.

Providing adequate tank room, a good water temperature, clean water and a healthy diet goes a long way to making your Piranha grow healthily.

Typically all Piranha grow quickly for 6 to 8 months, reach a certain length, and then slow down. The speed with which they grew per month, becomes their annual growth rate.