Assuming that the room is very well insulated (priority one) and your tanks are up to the temperature you want, It will not take much heat to maintain the air temperature and hence the tank temperature. The greater the volume of water you have ,the greature the heat sink you have. When I was breeding heaps of fish The ammonia in the air became a problem so once or twice a day, while working in the room, I used to leave the door open for a while to get an air change. It does not take much heat to get the air back up to temperature. Serious breeders and importers generally load their fish tanks up to the max to reach an economy of scale. Can be risky but profitable. Your overheads in a fish house are about the same for 100 or 10000 fish.