I bought a 250w submersible heater, i have a 50 gallon rubbermaid stock tank that i will bury in wood chips and top dress in mulch, plan to put a foil face foam sheet over the top overnight. I think it'll work, my question is how cold is too cold? 10..5..-5? I mean, I figure they are good about knowing when they are hungry, when they need calcium, when its time to go to bed. Do i even not worry about it and let them choose all the time?