I started out with 3 pond snails that came on the plants I purchased. I know they are considered pests by some but they don't bother me. I knew they would mutiply but I have to admit I am surprised how quickly it happened. I now have too many to count plus tons of eggs. I noticed a few of the snails have died, leaving empty WHITE shells on the bottom of the tank. Is this to be expected with so many or should I be concerned?