Hi,
I'm going down a similar path. For some reason I can't get search for "PID" in the English forum to find anything, even though they are there. But seaches like +setpoint(>controller<PID) find some useful matches.
Here are two informative ones I've found: ( The first contains a somewhat definitive response by AquaComputer )
Aquasuite 2012: Set point controller question - English forum - Aqua Computer Forum
PID controller tuning - English forum - Aqua Computer Forum
Good luck !