Aquarium heater questions.?

I just bought a mini elite submersible5 25W aquarium heater. I have my two 8 "pacu and my 7" plecostomus holds a 5 gallon tank to my 500 liters tank is fixed. Anyway, I was wondering where I should have the knob on the heater to maintain these fish comfortable … like to be within 78 to 82 degrees of water to grow, prosper, stay healthy and be happy … so if the command is to maintain this temperature range? Low, medium or high? Please help: D Thx in advance:)

Better to have an average (80). All values fall between 78 to 82 should do though. But you never know, the environment around the tank can always cause a slight change in temperature and whether they or too low or too high the environment can push it out of the border. So to stay in the middle.

CO2 Yeast Injection and DIY Diffuser – 30 gallon