Well Ghoster from everything i have red you don't have enough space for them to successfully breed. but you could try suddenly raising the amount and how often you feed them and do water changes every three days or so i read that this causing them to burn of a fatty layer and also helps in inducing eggs. But you will also read that it is very rare to be able to breed piranha but it is possible.
I've had reds bred in 40g, so space isn't the biggest concern.
reds are fairly easy to breed actually that's why they are the most common piranha in the hobby. most of them on sale in lfs are breed in captivity.
