That's one AWESOME tank...Do you have anything under those big rocks other than small substrate? You can place a plate with a lettuce leaf overnight and take out in the morning. It will have snails all over it.
Oh yeah....what wattage are you using in your lights and are you using a co2 system ???
PS when ur p's grow to 6"+ u need to get a bigger tank, nice tank and all but, 15 p's is to much for a 120g, the rule is 20g per piranha, so either plan on buying a 300g tank, or sell 10 of them