Personally, I would not run more than a total of 1500 watts on a dedicated 20 amp circuit. If your pumps, heaters, and powerheads are going draw more than 500 watts I would seriously think about setting up 2 dedicated 20amp circuits. If you have 2 circuits you can split all your equipment up so if one circuit breaks when you are not there all your pumps, lights, etc. will not turn off causing a disaster.
Example one mh, one heater, one ph, and one pump, on one circuit, the rest of equipment on the other circuit.