Thank you. I had considered this and you may be quite correct. What happens with the assassin snails when the clean out the others?
Well I think they are quite pretty and have left them in and have same amount still (several moths later) that I purchased so they don't breed like crazy. It'll take a while for them to get rid of all your snails but once they do they'll just eat leftover food that you feed the fish. But nitrates are rough on discus so i'd try to pull out all you can see of the pond snails( I promise there are tiny baby ones in your substrate for the assassins to eat).