The easiest control system is to set the number of hours/day that you pump water through the solar absorber with the pool pump timer. My only problem with this was during cool or cloudy days when this just cooled the pool.
Our Solar Pool heater used 3 8" x 12" fuel bladders ( 288 ft^2 ) that water was pumped up to and drained out into the pool. This unit raised the pool temp (16,000 gal) by up to 5 deg F / day mid-summer with continuous pumping during direct sunlight hours. This is not as efficient as the parallel tubing style that guarantees an even flow through the entire collection surface.
With todays electricity prices you would definately want a sealed heating unit so that the water descending from the rooftop pool heater helps pull the cool water up to the heater.
Most pool heater solar absorbers won't heat the water to more than 15 deg F above ambient with no H2O flow, so it is safe to not pump H2O when the pool is up to Temp.
A more sophisticated control system uses a differential temperature controller such as from Pentair.com that enables pumping when the collector temp is higher than the pool temp and the pool temp is below the set temp.
