Are there plenty of hiding spots in your tank? I don't see the clown loach as being the instigator, since they are generally very friendly, if a bit curious. I would look into trading the pleco in for a smaller one, since at 6 inches long it's gonna be getting a bit too large for a 20g tank. A smaller pleco would also probably be a bit more mellow. And again, make sure there plenty of hiding spots. The clowns will swim wherever they please, but when they're not active, they'll go back to the same location every time.