the right temps would be 76-82f...the higher the more active they are, and the more aggressive they get. the more active they are the more they will eat.
if you have a wet/dry sump you can put the heater in there.
What size tank are you keeping Piranh'as in? Tanks that are large enough to house Piranha's will usually require 2 or more heaters for even heating. Anything 4-6ft will require two 200 watt heaters, all the way up to two 500 watt heaters.