I have a 120g tank and a 40g breeder sump, with all the rock and sand, I figure I am at 120g total between the two. I want to run two heaters in case on goes out the other is on standby, or using two to heat the water. Question is, what is the best and safe way to run two heaters? Should I get two that would heat upto 75g or 1 that heats at 120g and a back up at 120g. Thanks for the help!