With piranhas its minimum of 20gal per fish IMO, 25-35gal being ideal, so for an avg sized 10" rbp thats a min of 2gal/inch, 2.5-3.5gal/inch ideal.
Long story short the more active the fish, the more gallons, I always try to give them as much as I realistically can, especially with predatory fish who are most often times messy eaters which poo alot!
My baby rhom is 3" and has 90gallons, so that's 30gal/inch, which, is extreme but its paying off with growth and health!
My shoal of (5) 6-8" rbp have 125 gallons, that's 25gal/fish, avg size of each rbp is 6.5" x (5) =32.5" TL / 125gals = 4 gallons/inch of fish in my tank.
If I had min requirements for rbp (20gal/fish) that would be 6 rbp in my 125, making it about 3gals for every inch of Piranha.
So imo 3 gallons per every inch of rbp IS THE ABSOLUTE MINIMUM and 4 gallons per every inch of rbp is better, but obviously more the better!
there are more variables in this equation, like footprint, filtration, bio load, decor etc.
Easy to calculate: add up total inches of fish in your tank, divide the volume (gallons) that will leave you with gal/inch of fish. (TL/volume=gals/ inch)
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