Red belly piranha in 85 gallon tank?

I've got an 85 gallon fish tank with 2 marineland emperor 400 power filters that do 400 gallons an hour each.. I'm interested in doing a red belly piranha tank I'm wondering how many could I fit in there? Even when they get large? I've herd I'd have to do a 25 percent water change per week is that correct? Please provide as much information as you can. Thanks!