Well, are you aware the black kuhlis rarely breed in captivity? People who attempt to breed them have much difficulty and most success stories come from them doing their own thing while hanging out in an under gravel filter plate. The black kuhli has even less success than the striped version.
As for keeping one, try keeping a group. They are quite social and really feel more at ease with more and more kuhlis. I have kept several species in this genus, all were wonderfully social and adorable. I wouldn't suggest under a 20g tank, and be aware that some people never see theirs. They are more likely to venture out willingly during the day if they are comfortable which means a good sized group and lots of hiding places. Don't be surprised if it takes a bit for them to venture out. They are not too picky of eaters, but I suggest sinking food.