If it's attached to the tub cleaning the tub is gonna be a hassle which is why it is better to have your heat source attached the the shelf of your rack regardless of the type of heat source you chose.
Heat tape will be more cost effective compare to UTH, and it can also be wired in parrallele which allow you to have to deal with only one plug for the entire rack whereas with an UTH on each level you will have to deal with one plug per level.
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