I was using them to control a 4 ft tank divided into 6 parts with a bare tank bottom and a below tank heating mat of 300 watts as well as a 4 foot planted tank with sand media and heavily planted, with an undertank 300 watt heating mat. The control on the bare tank was pretty good (hysteresis about 1 degree), but the sand I discovered needed to be evenly spread and not too deep or the hysteresis extended considerably (probably about 5 degrees)