Well, I don't test my water every week. I just tested it right now and amm is 0, nitrite is 0, nitrates is 5.
Ph is still holding at 7.8, but thats what the levels of the water are straight from tap.
If your local water fluctuates, then you should take the shells out, but as for left-over snail bits in the shell, my guess is the shrimp take care of that because I don't see anything in the shells I pulled out, and they don't smell (from decaying flesh inside), so shrimps probably play a more crucial role in this setup than I thought.