I currently have 7 tanks going (5g, 8g, 10g, 29g, 29g, 75g, and 90g). I started many of the tanks around 7-8 years ago. I have never used a heater. It's simply not needed.
Consider first the room temperature where the tanks are located. If it's around 70-75, then that's what the water's temperature will naturally gravitate towards. Then add in the temperature of your lighting. Unless you're using LEDs, the lighting will throw off some heat, sometimes a substantial amount of heat. That will serve as a "heater" for the tank. If you use hot lights for a long photoperiod with a canopy, you may find yourself having problems with the water getting too hot. You certainly don't need a heater in that situation.
So I first suggest that you use a thermometer in the tank so you can check the tank's natural temperature without any accessory heater. Check it in the morning just before the lights come on and in the evening just before the lights go off (you can do a midday check, as well). Record this so you can see the pattern. Then you can determine if you need a heater, and if so, how many degrees you need the heater to raise the water's temperature. That will then tell you how powerful a heater you need.
Just know that the idea that all aquariums need a heater is untrue. Let the water temperature tell you what it needs, then fulfill that need. If there is no need, then let it be at that.
