Well if you want to keep the glass tank then I would suggest making some changes to see if you can make it work.
Can you describe some more about your setup:
- do you have an under tank heater on a thermostat?
- sounds like you're using a heat lamp to warm the air in the tank. how many watts?
- how are you measuring your temperature? do you have a heat gun, analog thermometer, digital thermometer, etc?
- is the temperature in your room typically 70°?
Some things that can help with retaining the heat are putting insulation around 3 of the sides. You can also partially cover the top, which is typically done anyways to help retain the humidity.