VirtualWire and/or radiohead for RF433 communication.
This second arduino activates a solenoid in the CO2 supply to the aquarium and thus lowering the PH.
Seperate code works fine. Together RF433 still functions but PH gives "inf"
Softserial I can't remember since that was a year ago but remebered it gave the same issue so thought it might be relevant. The more info the better right.