i wouldn't recommend it, clown loaches may grow up to about 6 inches and may tend to be really active.. plus the fact that its better to keep em in schools..
Actually you are wrong, clown loaches over time get to (and a long time at that) be about a foot in length, and your also right, they like to be kept in a school. maybe something for that huge tank your gonna need for your ray and black gohst knife.
Well of course they will seem fine in a 20 gallon when they are tiny. As would a shark. But a little research into their adult size will immediately show how that is not the tank for them.