Lgrey23 has great advise. I'll just add a little bit of my experience with cool room keeping to the mix. plastic tubs are not good at holding heat they basically will have cool end and ambient temps the same or very close to the room they are in. There is no insulation value to start with and the options to heat the cool end are limited as emitters and lights are out of the question. That leaves a second uth and thermostat and then there is still ambient air temps to deal with.
I'd stick with the tank. Personally I'd try to insulate the tank and add a secondary heat source. i like using fluorescent lights for that as there is little risk of burns and over heating as they don't become much warmer than mid 90's enough to heat the air up and cool end but not enough to cause trouble. It is going to sound stupid and is not intended to belittle you but people forget basics sometimes. Heat rises. The big trouble with tanks is the open top. one option is is to side turn it or cover the top with some insulation, leave a generous air space on either side to promote air flow.
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