It's absolutely the Clarkii!
I just fed some chopped raw prawns, and he came to grab one, and whenever another fish was going for the food too, he'd click (you can see his jaw go) and grab the food. The click startled the other fish who then let him have the food.
It's blatantly obvious it's him now I know where to look.
Wow, learned something new, that's really impressive. It's quite loud too!