Have you ever visited the tropics and dropped your feet into the water? It is nice and warm and rarely changes. Ocean water in an area does not vary greatly throughout a day or even a week. Your tropical fish tank temperature needs to maintain the same kind of rocksteady consistency.
As a good starting guideline, your tropical fish tank temperature should stay within a range of 22 to 29 degrees Centigrade, 72 to 84 degrees Fahrenheit. Some species of tropical fish require much narrower temperature ranges in the 24 to 25-degree centigrade range.
The issue which catches a few new hobbyists of the guard is temperature consistency. They look at the range above, decide they do not need a heater and let nature take its course. Their tank moves wildly from a very cool 72 degrees at night to a very warm 84 degrees in the middle of the day. It sounds fine since it is all within the proper ranges. For the fish, it can be very distressing. They are not used to wild changes in heat, even within their preferred range.
What you really need for your fish tank is a good heating system which is controlled by a thermostat. The temperature of your fish tank is always monitored. When the temperature drops only a degree or two below your setting the heater kicks on and slowly heats the water back to the optimal temperature.
Keeping the temperature in a narrow range is the best for the health of your fish. It reduces the onset of many fish diseases.
Overheating of the tank can be an even bigger worry. Too often new hobbyists put an aquarium in an area which gets direct sunlight, turn off their air-conditioner, and then head off to work for the day. They come home to find fish floating on the surface. The direct sunlight can take the water which was sitting comfortably at 80 degrees up to 90 degrees or above very quickly. Oxygen in the water becomes depleted and fish die. Keep the tank out of direct sunlight, especially if you plan to shut off cooling on hot summer days.
If you do not want to be worried about keeping such a close eye on the temperature you will need to choose fish which are very temperature tolerant. Even these fish do not tolerate rapid temperature changes. Some fish, like a goldfish, are capable of tolerating temperatures approaching freezing to nearly 90 degrees Fahrenheit.
The best advice is to always check the preferred temperature ranges of the fish you would like to own, then buy heating systems to work with your aquarium to match the volume and needs. The cost of a high-quality heating system with a thermostat is very low, and a great investment in having the best aquarium possible.
The only other important note on tropical fish tank temperature, make sure you match fish which tolerate the same temperature ranges. You do not want to have fish needing a tank towards the upper-temperature range, and one at the lower temperature ranges in the same tank.