Snails are dieing

I have noticed a lot more snail death in my tank within the last couple of months.  There are always a few snail deaths every once in a while and I see the shells at the bottom of the tank; usually an astraea that can't right itself.  But within the last couple months I have lost 90% of my snail population.  I went from over 15 snails to 3.

I have noticed tiny cerith looking snails on all the dead snail shells.  Also they are on the edges of every alive snail in my tank.  Could they be some sort of parasitic snail?  They range from 1-3 mm (approx) in length.  Have conical cerith like shells that are completely white.  Unfortunately I don't have any pics.  Anyone ever seen or heard of anything like this?

Nope no clams.  I take it they eat clams as well right?

Where could they have come from?  I hadn't added anything new to my tank for a few months before I noticed them.  Any way to get rid of them or should I just let all my snails die, leave my tank snailless for a few months, and then restock?

Another few questions.  I have various snails that have "grew" out of my LR.  I believe one is stomatella (half-shelled slug) and the others are tiny black and cream spiral nocturnal snails (look like sundial snails but eat algae not zoas).  Will these snails be killed off as well?  Also the one snail that has none of these parasitic snails on it is my large turbo.  It cruises around all day and I have yet to see one on it.  Are they immune to these things?  Should I switch to turbos?

Hmmmm... I may have to consider a six line in the future.  I am going to be getting a bigger tank and do not want to transfer over these pests so I should try to get rid of them now.  I want clams in the new tank so this definitely needs to be cured before the transfer.

I may get a six line but wanted a mandarin so I will probably leave the six line as a last resort.  For now I think I'm just going to let my snails die out.  Wait several months and restock.  I never have any algae problems (even when I got down to 3 snails) so I should be fine.  If I start seeing too much algae I can always get some hermits (even though I hate those stinkin shell stealing sobs).  Can these pyram snails survive very long w/o other snails to feed on?  Would several months of no snails in the tank be enough?