Hi,
I`m not sure how many inches or cm a 20gallon fish tank is, but I`m sure it`s much too small to get a proper heat gradient and a decent depth of substrate (the substrate would help with the humidity and keep the monitor hydrated).
You need to know only two temps; the lowest ambient (air) in the coolest parts @ approx 21 to 24c (72 to 75f), then the SURFACE temp at the basking site @ between approx 50 to 60c (120 to 140f), no other temp matters.
The humidity range should be between approx 50 to 70%, it may even be slightly lower around the basking area, but that`s fine. You need to understand this is a tropical species, not arid (dry like a desert).
I think you need to upgrade the enclosure asap if you want the animal to live a long, healthy life. The health problems start from day one, not at some point in the future (no disrespect).

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