A 20 gallon high tank will have a light that sits on top of the tank, about 14-15 inches from the substrate. One T5HO bulb, with a fair reflector, and a real HO ballast, will give you about 60-70 micromols of PAR at that distance. Two bulbs close together will give double that - say 120 micromols of PAR. A single layer of fiberglass window screen will reduce that to 70 micromols, which is still high medium light. To avoid most algae problems with that much light you need to run the CO2 at the optimum bubble rate, where the fish aren't distressed, and more CO2 doesn't cause better growth of the plants. You can't rely on a drop checker for that. You need to slightly increase the bubble rate, observe the plants and fish for a few days, looking for improvement in plant growth and no fish distress. If you get some plant growth improvement, repeat this, until you don't, then back off the bubble rate to the previous setting. This will be a changing target unless you keep the plant mass stable by pruning regularly and often. You also need to be sure the water surface is all rippled from water currents just under the surface, so the dissolved oxygen in the water is also kept high.
I think it is much easier to just reduce the light intensity some more, down to 40 micromols or so, where CO2 isn't as critical.