Ok first, your not feeding your hermits enough food. I have three and never did they kill any of my snails.
Second, a Nessarius snail is good for your sandbed, it basically lives in it but will pop up from time to time to say hi
and they are about $4 a peice, just be sure to keep your ammonia lvls down or just about everything will die but the hermits.
And thirdly, your tap needs to be tested at you LFS
before you use it but if you do use it dont be too extensive or excessive with it b/c eventually it will make your water lvls go cuckoo. Try using it only for quick/small top-offs, other than that like for evap. replacement I use Distilled, its much cleaner and better for use in bigger quantaties.
But your Hermits shouldn't kill your snails unless they are hungry, geez lol I had snails hitchin a ride on the back of my hermits lol starfish too (astrea) but yeah try feeding them more. How many do you have? What Specie are they?