The one on the left is the introduced Ramshorn snail and the one on the right looks like the common pond snail. Larger ones like that probably don't get eaten by any but the largest PS and RES. But there are always smaller ones that will...

Darters do love snails... Three species of darter are adapted to pond life. The swamp darter, tessellated darter and Johnny darter. I am in the process of building up the darter numbers in my one pond right now. If you have sandy or rocky areas in your pond, the tessellated and Johnny darters should do well, the swamp darter is a little less picky but rarely gets larger than 2" where as the other two species will push 4".

None of these species are available commercially but are all very common and can be collected in most streams in the east(tessellated darter) or in the Mississippi drainages(Johnny darter) or along the coastal areas(swamp darter). If anyone is interested PM me and I will give you more information on them...