will be fine. The only problems with U.T.H.s on glass is if you have them under the tank with no circulation. If you place them on the bottom of an aquarium and don't use a small spacer on one or two corners of the tank, there is no circulation and the heat is trapped. This will crack the bottom the tank. An U.T.H. on the back of an aquarium has plenty of circulation.
Exo-Terra U.T.H.s put off very little heat compared to others. The 8W Exo-Terra heaters hardly produce any heat in my experience. Zoo Med U.T.H.s actually produce heat. Zoo Med heaters come with the spacers I mentioned.
Phil Goss