Hi, I recently set up a 125 gallon which I transfered my oscar and his filter over to. I currently have one aquatop 500watt heater in the tank which is rated up to 150 gallons. I also have a couple thermometers in the tank to check the temp and make sure the heater is working. I have a second of the same heater and was curious if I should run two heaters in the tank. so far the heater is preforming quite well with no changes in temp acording to the heater thermometer and stand alone thermometer. If yes should I stick the other identical heater in there or run a smaller one along side the one in there now? Thank you for all of your help im open to everyones opinion.