My Piranhas....

[MikeyBizzle]

Then a 75 could have 4 fish and 5 in a 90....

 

[HarleyK]

do you have any other fish in with them?

You mean feeders?

 

[jp80911]

75g and 90g have the same footprint so I would say the amount of fish to be kept in there should be the same, it's not always about water volume.
if you think they poop too much it means you feed them too much, how frequent do you feed and how much do you feed per feeding? cut back on feeding and they will poop less.

 

[fish042099]

I think 1 piranha for 20 gallons, so you should get a 75 or 80 gallon tank.

 

[Jamo3030]

FX5....