Have you tired to insulate the glass tank? Things like thick paper, foam, carboard, etc. can be used to insulate the tank and boost the temps.
You are referring to a tub alone and not a rack system?
A tub can present the same issue as a glass tank in a cold room. Some that use tubs have to use heat lamps because the tub is not well insulated.
You may be giving up before you exhausted your options.
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