We had a HUGE snail problem in one of my daughter's tanks. I put in a few thinking they were cute and we ended up with hundreds!!!!! I was scooping like crazy trying to get them out. I do have several dojo loaches but, they are large and used to a big tank and I didn't want to start tank hopping my fish and end up with any problems either. I asked for advice and began monitoring my daughter's feeding of her fish. She was feeding a bit more than was appropriate for the fish she has.... we cut back on the feeding and the snail population is now manageable (maybe 5-10) without any scooping. What seems like a little extra food for a fish, is a smorgasbord for snails and they will reproduce like crazy. Try the lettuce but, maybe cut back on the food for a bit and see if it has any effect as well.