I bought a Marineland 100w submersible heater for my 30 gallon breeder tank and the settings knob has to be set at 73degrees making the tank range from 78-80degrees. Also I paid $40 at my LFS(rape).. I dont mind supporting my LFS, but Drs. Foster and Smith have it for $18.40($24.40 after shipping). So as one Marineland heater rated for a 30 gallon tank varies 2degrees and has an offset on the knob.