As SDA was talking about......When I did have glass tanks I would use a CHE (60 watt) in a mini dome for a 20gallon long in conjunction with an under tank heater......both heat sources on thermostats.
Then (to address the humidity problems you're going to have with this setup) I would cut a sheet of 1/4" Lexan (a type of plexi) to cover the screen top with the exception of a large hole to clear the mini dome and leave maybe 1/2" of clearance around it. So, if your lamp fixture is 6" in diameter, cut a 7" circle in the lexan. Your lamp fixture should then be sitting directly on the screen top. I SPECIFICALLY recommend lexan as it is relatively heat resistant. I know for a fact as I have taken a blowtorch to it for other projects and it does not like to hold flames and it can take incredible amounts of heat without melting or offgassing. However, take care to keep the lamp fixture centered and not touching the lexan just to be safe.
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