It could be a long battle? You could try the lettuce, but I don't know how affective it is. Clown Loaches should be kept in groups and will get to large for your tank. Even if you did they probably would not get all of them. You could frustrate your self to death every time you see one picking it out, or you could buy chemicals to kill snails but you would have to be careful not to harm your fish and get dead snails out before it pollutes your water. There are other species of fish that will eat snails if you look around. I have foolishly got snails a few times because I didn't inspect or quarantine plants enough. Both times I had a great flashlight and some patience and found them all after a few days. Good Luck