Question
I have a prefab pond that holds approx. 160 gal. I have added plants from the local river and lakes to it as well as snails, mussels, and fresh water clams. I have an algae eater that I bought when I got my gold fish all of which are now about 6 to 8 inches long. My snails keep dying, and I find empty shells, is the algae eater eating them?

Answer
There are several reasons why your snails may be dying.  As always, check the pH, salinity, and Ammonia to ensure that they are all in check.  Unlike goldfish, which thrive with a bit of salinity, snails cannot live in the presence of even the slightest amount of salt.  Ammonia and pH issues can also be hazardous.

The other causes could be starvation, parasites, bacterial infection, or being eaten by a goldfish.  Starvation is actually a very common cause of snail death, and even though some are dying there may hundreds surviving.  If this is not the case, and there is plenty of snail food (algae, decaying plant matter, etc.) and it is the majority of your snails that are dying (not just 1-20%), you may need to look further into the cause.  Parasites are common in almost every species on Earth, and are especially prevalent in streams and lakes.  It will be nearly impossible to eradicate their parasites, should that be what is wrong.  A bacterial infection can be taken care of by maintaining a stable pH (done by maintaining a proper carbonate alkalinity) and keeping Ammonia at the lowest concentration possible.  Depending on your water source, monthly (or weekly) 20% water changes should ensure these parameters.