Proper levels for Piranhas

I just set up my 55 gallon tank yesterday, and i put 30 guppies in there to speed up the biological process a little as well as putting the bio filter additives. I was just wondering how long it would probably take, as well as when its all said and done, what my General Hardness levels should be at, and if i should use chemicals to lower it. Its at 6 degrees right now on the second day of the tank being set up.