I have the 501 on my 20L and it works great. Let me mention though as theirs only my congo puffer in there and just has a few small pieces on manzanita wood, anubias, and angustifolia. I would imagine if it was planted heavily you might start getting dead spots but for an average set-up or medium-low lying stuff I would say go for it. Have a 501 on my 12 long as well and my cardinals get tossed around a little if they're within 6-12" of it so it definitely pushes water fairly nice. Never tried the 511 but for only $10 more (with sale) It's a great deal. I would imagine that the 511 would be great for heavily planted 20L. Def would need a good swimming fish or direct most of the flow if your going to go with the 511 in a 20L though.
75G, 29G, 20L, 12L, 10G, 5 Nano's, 3 Betta bowls
12G Rimless Club
*Everybody has too much of something...me? I have fish tanks.*