You need to figure out a way to regulate the temperatures, not just add more heat. Especailly as an open glass tank is going to let tons of the heat escape. That means you need to get a second UTH on a second thermostat. That's kind of the bare minimum deal here. They don't use that much power - at all.

But again: cold side 80. Warm side 90. Give or take maybe 2 degrees, but you need to get those temps up to that, NOT above that, or below that.