How many watts of heating are you currently running?
What is the ambient room temperature in the room the tank is in?
Your initial post for a 500w heater for an 800g system is extremely far from sufficient, even if you keep your room temperature at 73 degrees. I hope you've got other heaters in the tank and this 500w request is to replace one that might have failed already.
I asked for 500 plus because I know I would get a few people offering heaters and I know I will need a few units I'm currently in possession of 3 300w heaters but wanted to see what was available the tank is currently in a climate controlled room so the reason I'm asking for another heater is so my current units are not working as hard. No longer looking to use 300w units looking for bigger stuff
Heaters are the number 1 piece of equipment to fail in our hobby. I applaud your decision to use multiple heaters for redundancy. Good move. I don't agree with using used heaters. You are flirting with danger. IMO you should use nothing less than new heaters.