Okay, I will explain why I am considering swapping my heater. My 100 w heater seems to not go below 24 degrees, it is an aqua vital one, it is placed in the sump in a very small return section. Theoretically I should need a 200w for this tank, however as it is heating a small volume in the sump then pushing this water into the tank it seems to heat it above the 24 degrees it is set at, maybe this is a fault with the thermostat or with the numbering on the heater. My 50w which is a visitherm (much better heaters IMO) does go down to 20. It seems if it is hot to compensate I lower the temp, always have done, even in my 55g freshwater which has an aqua one it seems to need some compensation either side of the required temp in this house during summer or winter. In winter if I do not raise the temp on the heater a couple of notches (Our house is heated by a coal/wood fire, is stone built) the temp in the tank can fall to 22 degrees from 28. To keep the tank at a constant temp, I adjust the temp setting on the heater, I have done this for the last 3 yrs, I always thought it would just heat to the temp set and then switch off, it does not seem to work like that however. I have also tried several different heaters and several different brands so it is not a broken heater I am dealing with, it also happens no matter which tank. I have a feeling in summer the tank can manage with a 50, but in winter it may need the 100 or higher. In the 55 it has 200w, in summer could prob get away with 100w but need higher in winter.
I believe it may be something to do with the way the heaters are set to work. They are supposed to heat water slower, eg. If fish tank is 22 degrees and it should be 28 degrees it will not heat the full 6 degrees in one go, it will do it slowly so as not to stress the fish, this is deliberate I believe in the manufactur of these heaters. The other thing is with the actual wattage the heater is and the tank size it is supposed to be used in. A 50w in a 200 litre tank will struggle heat the full size of water to a point where it is over heated, a 100w in the same volume of water will maintain this temp easily and could potentially overheat it, quite easily. I do not believe they are accurate enough to maintain a temp set on the thermostat. A more expensive one with temp probe just might be, these glass ones, I doubt.