Agreed. I definitely wouldn't put a fish tank of any size on top of a rack. You might get away with something 1-2 gallons or less full of water, but more than that would be a bad idea. (Not to mention what could happen with spilled water from a water change.. too likely to short out the heat tape or drench a tub.)
A few tarantula tubs would probably be just fine, but water is heavy (~10 lbs per gallon is the aquarium-hobby standard estimate) and glass tanks add substantial weight as they increase in size due to the glass as well.
Sent from my SM-G950U using Tapatalk