Depends on how deep you want the substrate and the tank configuration. For my 55g, I ordered 5 20# bags and ended up with an average of 3" of substrate (2-4" in the tank since I tend to slope my substrate a lot). I could have easily gotten away with 4 bags/80#, but wanted some extra depth for sword plants and I like a deeper substrate. Generally stated is 1.5# per gallon, I always liked more. With no plants, you could go as low as 1#/gallon, since it's a 20 long, then you'll need more per gallon since they have more bottom area.
Oops, never really answered your question - I'd say if you like a deeper substrate, go with a minimum of 2 bags.
Last edited by DKRST; 03-05-2011 at 03:45 PM..