Planted Tank Guru
Join Date: Sep 2006
Location: Northern California
Well, first off, your 20L is 30" long. T5s come in 24", 36", and 48" (roughly). Your T5s will either be too short or too long for the tank.
I'd stick with a Coralife fixture...I don't know which has the 2x18w, but there is a 1x65w 30" fixture that might go well with your tank, especially since you have pressurized CO2.