Congo tetras are a large characin (tetra) and IMO too large for just a 20 gallon. Any of the smaller to mid sized tetars will do just fine, seeing as Kribs are from soft water rivers anyway. I personally like serpae tetras, but thats just my own preference. You just want to pick something that can with stand the abuse breeding kribs can dish out, so I'd steer clear of long, flowing fins and super bright colors.