the hydro thermostat is what controls the water temp. it goes up to 110 deg. i set at about 85 so the water stays that warm and the fc stays at a constant 73.9 deg so seems to be working great first time ever doing this so we'll see.....you cant mess it up believe me its simple, you can buy the therm. on ebay allready prewired for this for under 50 bucks