Temperature Controller For A Tropical Fish Tank ?

Hi Peter, these threads https://forums.thinkfish.co.uk/fish-tanks-and-equipment/heater/ and https://forums.thinkfish.co.uk/fish-tanks-and-equipment/tank-heaters/ might be helpful as a "starter for ten", and there's a useful tip in this thread re a specific make of heater https://forums.thinkfish.co.uk/fish-tanks-and-equipment/e-type-fluval-heater/.

I use a central heating controller with an external sensor... because I work for a company that makes them. There are other reasons though, they (or at least ours) have advanced control algorithms that adapt to the heating characteristics and proportion the heat evenly rather than just cycling on/off. Any external controller will however have advantages over the heaters own thermostat because they are not 'the thing' that is providing the heat and controlling it at the same time and they give an additional level of safety to overheating if a thermostat fails.