I didn't change the temperature of my heaters but I did have them set on 18-22C depending on what fish were in the tank. As the weather cooled the water temperature dropped to 18-22C and sat there over winter. As the weather warmed up in spring and summer, the water temp went up too. In summer the water sat on around 30C (86F) and dropped to 18-22C (64-72F) over winter.
I had a lot of rainbowfish from Australia and New Guinea, and native fishes from the south-west of Western Australia and they all experience temperature fluctuations during the year. In the south-west of WA the water temperature can fluctuate 30C (from 5-35C) during the year, whereas in the north of the country it might only vary 10-20C during the year. I also had barbs, cichlids, catfish and tetras. Some of the fish like barbs and natives from the south-west didn't have heaters and the water would drop to 15-16C over winter (sometimes lower than that). When spring hit and the water got to 18C the fish started breeding.
When keeping fish at lower temperatures you can reduce the feeding and monitor fish for fungal infections. I had a number of bottom dwelling fishes (Galaxias occidentalis, G. truttaceus and Bostockia porosa) from the south-west that would get covered in fungus and stick to the bottom of the tank. I don't know why it happened but it did every year.
In the wild fish will be exposed to varying water temperatures with warmer water found near the surface and cooler water at the bottom of the river, creek or lake. This is more for stationary waterways and less in fast moving water. I tested temperatures in pools and waterways in the south-west of WA and there could be a 10-15C difference between the surface and bottom. I was in a pool on a golf course down in Walpole (Western Australia) and there were baby salamanderfish (Lepidogalaxias salamandroides) in the shallows. The water temperature was 30C in the shallows (1-2 inches of water around the edge of the pool). I tested the temperature at the bottom of the pool (it was about 4 foot deep) and the temperature was 15C. The adult salamanderfish were found on the bottom while the babies were at the top in the warm water. The warm water helps speed up their metabolism so they grow faster and there is more food in the warmer shallower water, which also gets more light to encourage single celled algae.
I tested smaller pools of water in different areas down that way and some were only a few inches deep and the water temperature was around 35C during summer. Some of the smaller pools had no fish (they appear to have been fished out by birds, there were bird footprints by the pools) while others had a variety of juvenile and adult fishes in them.
I tested moving water (creeks and streams) as well. Some were fast moving while most had a moderate or slow flow. The water temperature in these flowing creeks was noticeably cooler than the stationary pools and most of the moving water had temperatures in the low to mid 20s Celsius during summer and 5-10C in winter. The stationary pools and flowing creeks contained the same species of fish and all the areas tested were within a 50km radius. So even in the wild in the same location, there are temperature fluctuations in the water temperature and the fish are exposed to these fluctuations throughout the year.
