You have 3000g females 2 each in a 28qt tub? A female that big probably needs her own 41qt. Like was said above, the most economical thing to do is get appropriate size tubs with UTHs on a thermostat. You will be out about $40 each for a tub and UTH, and $30-100 for a thermostat.

What are you heating with now? I do not want to armchair quarterback what you have going on here, but why do you have these big females and not have appropriate housing? Unless I am understanding your first post wrong.