Aggression is hard to eliminate in 20L. That is why I prefer 4' tanks. The length allows for a better retreat and most substrate spawners prefer a 24" territory to defend. Therefore, a dominant male would stake out an entire 20L as his own and the rest would be getting a workout 
If you don't have access to a 55g I would try adding some clay pots with holes in them or some PVC pipe to offer more territory choices. If they are a pair the bond will form faster if larger weekly water changes are done as it will incite breeding.
HTH, Rich
If you don't have access to a 55g I would try adding some clay pots with holes in them or some PVC pipe to offer more territory choices. If they are a pair the bond will form faster if larger weekly water changes are done as it will incite breeding.
HTH, Rich