That is perfect for a 20 gallon. I have a 29 gallon and have a bit over 2 watts per gallon (65W CF
bulb) and do not inject CO2
and my plants are pretty much out of control.
1 yoyo loach, 2 polkadot loaches, 6 cherry barbs, 2 turquoise rainbows, 1 chinese algae eater, 2 harlequin rasboras; java fern, java moss, hornwort, Asian ambulia