I agree that the most important factor is the species being kept. Fauna found in large bodies of water are less accustomed to temperature fluctuations than those found in smaller bodies of water. For example, I keep white clouds in unheated tanks. In the warm months their breeding slows way down/ceases, but I often witness them breeding like crazy after water changes with slightly cooler water. This has not been my experience with more tropical species.
If you can determine that the fish you keep come from waters that drop in temperature at night, I see no reason why duplicating that could be detrimental in any way. Just bear in mind that the Amazon, African rift lakes and such are unlikely to have much in the way of nightly fluctuation.
Nothing good happens fast in an ecosystem.