I would have to agree with froghub that a 25 watt is not going to be enough. The 40-50 should be ok for a 10 gallon, what you want to do is cover half to just a little more then half of your lid with tinfoil and it traps the heat in. And yes it is fine to do this, just make sure you have an opening at each end to allow air flow so the air doesn't become stagnant. Some people will actually cover up to 2/3 of the top, I have kept all my pacman frogs with the little more then half covered and have never had a problem. I have a reptile/frog room that is at constant 75f all year round so I can get away with not having to cover 2/3. And the foil also helps with humidity a lot.