African brown knife fish can get up to 12 inches in the aquarium. I think you can fit it in a 20 gallon but IMO I wouldn't recomend it at all. I definately don't think it would be the best thing for the fish's health nor it's happiness. The african knife fish is a predator and will eat smaller fish in the tank. They also need slightly acidic water with a PH from 6 to 6.5. They don't like a lot of light and need rocks and caves to hide out in.
I really don't think you should add one to your tank at all. It would be a disaster. depending on how many tetras, barbs, and danios you have...maybe you could add some more to your schools of them instead.