It's hard to say if a 55W kit from ahsupply is too much for a 20g long low-tech tank (no, I do not follow this watts per gallon concept), because it's quite easy to lift/lower the kit from the tank to increase/decrease intensity.
I'm concerned about coverage though. The kit is only 22" long, so even if you center the bulb, you're looking at about 4" unlit areas at both ends, but it might still get covered enough if you raise the bulbs enough.
You can look into trying into trying the 24" T5HO retrofits with individual parabolic reflectors. Same deal with the ahsupply kit, but it has slightly better coverage being 2" longer, and you get less light lost through restrike because of the individual reflectors, but good T5HO kits/fixtures are always much more expensive.
I'll be setting 15g tanks (24"x12"x12"; same as 20g long, only 6" shorter) tonight with a single 24W T5HO bulb with parabolic reflectors, one bulb for each tank. I'm quite confident that should be enough for a low-tech tank, so I'll be able to tell for sure after a few months.
