Originally Posted by Sherlock23
Mostly all fish do. I don't have a heater for my tank ( I know I'm bad I know but Money is tight and a heater on thr bottom Of my list of things I need) but they are doing alright. I just keep adding hot water to the tank every time it's gets cold. But sometimes if you keep your house warm that sometimes would help without a heater but most fish do need one. I know cold water fish ( gold fish) would do alright without one.
Actually they do need a heater. And a constantly fluctuating temp is even worse than a low temp. Not living at the proper temp will reduce tropical fish life spans. Fish can't regulate their body temps like mammals can. The enzymes in our bodies only work at certain temps; this is fine as we can work to keep these temps steady. Fish have enzymes too that need certain temperatures to work. They can't regulate body temps (with a few notable exceptions), so these enzymes aren't working when the fish isn't kept around the proper water temp. Without proper temps, you are basically slowly killing your fish. There is some debate as to what exactly is proper temp for a betta, tho.
As to the OP's question. Fish don't need heaters, they need proper water temperature. I once kept fish that need a water chiller because the average room temp of the water was too high. What is the average water temp in your house? I'm talking winter and summer averages. There are some fish commercially available that can like temps around 75F.
Fancy goldfish (ryukins, moors, vieltails, ect) do best at temps between 73F and 78F. They can benefit from a heater, but it depends on your house temp. I don't need a heater for mine because they are in a warm room. The goldfish that don't need heaters (commons, comets, shubunkins) grow too large for normal home aquaria and do best in an outdoor pond.