Heat in one spot while the surrounding glass is cold can cause a crack in plate glass over time, but the hotspot has to get pretty hot.

Anyway, have you tried insulating the top of your tank with something like 3/4 inch foam insulation? I have found that insulating the tops of cages helps a great deal with heat retention.

You can also use fiberglass insulation behind the flexwatt (read the temp limits on the insulation however.) They use fiberglass insulation in small halogen fixtures to keep the heat going one way. This will keep alot of the heat produced by the Flexwatt from being wisked away by moving air. This is your problem: too much of the heat energy is going elsewhere - sucked away by cooler surroundings.

I don't think a light above will be as effective as you think, being that heat rises. You can try aiming a spot bulb at an angle hitting the side of the tank. Or tape a strip of Flexwatt on the tank side as well (at least during the cooler months). But I think the suggestions above will go a long way.

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