what are you trying to keep in that 20 Gallon Long tank with MH- have you though about heating issues, I'm currently using a Eclipse 3 with a 65w PC upgrade for my frag tank in a 20L, or how about a 30" PC Light?? 2x65w wouldn't that be enough, a 20L is only a food deep, I just personally think that a MH would be a bit of an over kill.
Danny