a 75 or 100 watt bulb is to much for a 20 a gallon but should work fine for yours. Bulbs have tank size suggestions on the box. One easy thing to do is get the 100 watt bulb and put it on a dimmer. This extends the life of the bulb and allows you to adjust it to the perfect output. Make sure you get a regular style bulb and not a spot style bulb.

As for resting them on the screen I always recommend not doing that. Resting is on the screen makes you move it and being the heat they can put out you run the risk of melting things or even starting a fire. Also moving them and bouncing them around shortens the life of the bulb. I recommend using lamp stands, big box pet stores $20. These allow you o raise and lower, extend and retract the fixture so you can point and direct the heat and light exactly where you want it. A lamp stand also keeps the lamp over the enclosure the entire time so the tank does not cool while the screen is off and the light is moved to the side.A lamp stand can also be used as a dimmer. To get less heat raise the lamp and to get more heat lower it. Its pretty simple.

To give you an idea of what it may take to heat your tank I will tell you about mine. Its a 40 gallon in a room that's a constant 68F. I keep the screen mostly covered and covered three sides with thick art paper. To heat is I have a 100 watt che on the hot side and a 60 watt bulb on the cool side. Both fixtures are at full power but placed on lamp stands hoovers a few inches over the screen. My che has been pumping 24/7 for over two years now. They are a bit expensive but when used correctly they really pay for themselves. The trick is to use the wire basket style fixture made for them as they do not build up the heat and burn up the unit. Also I never move it or bounce it around since its on a stand.