Should there be some remains if they aren't being eaten?
Yes. When a snail dies there will be decaying remains inside the shell accompanied by a very foul odor.
Do snails need any type of food?
In the wild, mystery snails eat algae, plant leaves, and anything else they find on the bottom. In captivity, they eat the same thing as well as fish food – a much more nutritious menu. They do best in planted aquariums and chew on plant leaves a little bit.
Mystery snails can "play dead" as well. You may see them floating at the surface of the water, then slowly release air bubbles and descend to the bottom. This is a normal behavior.
Be careful if ever treating the water with medications as some can harm snails.
I wonder if the algae eater died of starvation?
If your tank has any amount or traces of green algae in it, chances are that he did not die of starvation. I'm sure he managed to eat something in the short amount of time that you had him. Starvation of algae eaters occur when they have completely cleared the tank of all traces of algae and new algae doesn't grow fast enough to keep them nourished. Wafers also have the benefit of providing vitamins that are essential for their health, but again I don't feel this caused his death.
Buying from the big chains isn't always bad. For one, they have better prices, but sadly, they take little pride in the animals/fish they carry and it usually shows in the quality of health of the animal/fish. Smaller shops can carry ill animals/fish as well, but chances are better care has been given increasing the chances of survival.
A few things to take notice of when shopping for new fish:
1. Are there any dead fish in the display tanks?
2. Do you see any obvious signs of disease on the fish for sale in the display tanks - fungus, ick, sores, wounds, clamped fins, cloudy eyes, waddlely swimming, resting at the waters surface, resting on bottom of tank, lethargic?
3. Does the fish department smell "fishy"?
4. Watch the fish carefully for a few minutes, find one that appears to be healthy and active, then request to buy that particular fish, double check to make sure the sales associate caught the right one.
All tanks in pet shops (with the exception of some ma&pop shops) are running on one filtration system per type of environment. Salt, brackish, tropical, and cold water aquariums will each have only one filtration system (sometimes tropical and coldwater are combined as freshwater). This means that even though the fish are in separate tanks, the water is shared between all the tanks and filtered out and back into the tanks by one filtration system. So if a good number of fish appear to be dead or ill but are in a different tanks, still don't buy at that time.
-Kae