It gets better and better. Thanks to Rick for all this info.
I am running 2 heaters (500W & 300W) with separate controllers in the sump for 300G tank. Each Controller can handle up to 800-1000W but I decide to run 2 to cross check temp on both controllers and just in case 1 fail, the other back up. I place both titanium heaters in last chamber where the pump pushes water back with the returns on both ends into main tank and temp probes place in first chamber other end of sump where drain water from main tank. I am not sure tanks are not cover the top or lid but i cover mine and temp is stable so far.