It seems to me that as long as they're set to proper temperatures, there'd be no reason for the tank to crack.
To actually crack a pane of glass from heat, you'd have to have one side get a lot hotter than the other very quickly. Sudden heat changes. Or maybe if you're heating the entire bottom of the tank, the bottom pane of glass could expand enough to press on the outer four panes for something to break.
All really, really unlikely with the temperatures we're dealing with.
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