Unplugging your heater makes no difference. If you heater is set at, say 24°C, and your water is 28°C, then it just means your heater won't come on until the water gets to 24°C or below; it won't switch on if the water is at 28°C.
The ambient temperature is likely to drop fast at night, so you definitely shouldn't leave your heater switched off, or your fish might get too cold.
Don't forget that these are tropical fish we're talking about; they can easily tolerate higher temperatures for short periods of time. Just make sure you have enough water movement, as hot water holds less oxygen than cold.