I wasn't sure what to say since your parameters seemed perfectly fine. However, after reading this, this could be the problem. Fen/flubendazole is sometimes quite harmful to pest snails. It's really hit or miss. So in your case the effect seems to be killing them off.I did treat with Dog Dewormer, a few weeks ago, but I did not have any problems until now. Still no change in all of the snails. I am finding alot of empty shells.
Do a test, remove some MTS from your tank and just place them in a cup of tap and see how they do.