Apart from heating the water, a heater is also used to maintain a constant temperature and avoid too much change. It is well known that a fast temperature change can bring in infection such as ick. How is this controlled in nature in real life ? Someone please explain for me.
Good question. I have no idea, but I would guess that the water near the bottom would be less affected by surface conditions (sunlight, wind, etc). So if the fish gets too warm from the sun, it could swim to the bottom, where it's more comfortable.
Thanks everyone for the response. This has greatly helped me understand what happens in nature in regards to temperature. Once you know this answer, it appears very simple, however it is very important to know.