How long until my piranhas reach 6 inches? They say thats how big they should be to feed them mice. They are 3.5 to 4 inches. There are 5 of them in a 55 gallon tank. The water temp is 81, the ph is 6.9. I feed them shrimp, pellets, and krill. How long will it be before they hit 6 or more inches?