There are two ways you could heat your fish tank with propane. One is a real Rube Goldberg and involves using the drain tap from the hot water tank. The other is a small dedicated apartment hot water tank. In both cases you need to build heat exchange(s) which sit in your tank system somewhere like the sump. I built mine out of PVC. You need a small circulating pump and an electric valve that is controlled by your Neptune system. If you're going to go to all the bother of building it, you might as well build a cooling circuit using an old car radiator which will use up one more electric valve. One valve will be "normally on" and the other "normally off". The pump runs continuously and is either circulating heat from the hot water system or running thru the car radiator. My system keeps tank temp within 0.8 of a degree summer or winter. Use your fish tank heaters as backup and have them set slightly lower than your hot water system allows the temp. to drop to.
