Yes, tanks are VERY difficult to maintain humidity in. You would have a much better time in a tub. I realize you have substrate in the tank, just listing things you would need for a tub setup. It really is MUCH easier to maintain temps and humidity in a tub, hence the smaller dish for a smaller space. Are you using a thermoSTAT in addition to a thermoMETER? If not I would get one ASAP. Unregulated heat sources are a recipe for burns.
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