Try adding an air wand or bubbler to the tank to add more oxygen to the water. Since I have done that mine rarely venture to the top. Supposedly they go to the top and use their siphon when the water is low in oxygen.
I was thinking of this ............you said mystery snails and I was thinking of this I read about apple snails.
Apple snails that are found in the aquarium trade don't make high demands when it comes to water quality: they can live very well in clear, streaming, oxygen-rich water as well in still water, with rotting organic waste, containing almost no oxygen.
In general one should apply the same rules for water quality as with fish (ammonia, nitrite, nitrate etc) and the water should not be too soft. Like most snails, apple snails prefer calcium rich water. If the calcium concentration in the water isn't high enough (soft water), they aren't able to build a strong shell and become susceptible to shell damage, but even in good conditions, some snails still get little holes in the shell surface, especially in the older parts of their shell. This is a naturally occurring process and as long it's only at the surface, you shouldn't worry too much about it. In the 'snail disease' section, you can see an example of a snail with a damaged shell. Young and healthy snails are somewhat protected against this as the outer layer of the shell consist of a protein layer that prevents a breakdown of the shell, but damaged shells and shells of older snails are quite vulnerable to shell detoriation.
Warning! If the tap-water in your area contains copper and/or other metals, use one of these water preparation products that catch away those metals bofere putting the snail in the water. Apple snails are very sensitive for these compounds (especially copper). You won't be the first one loosing a snail due to this snail-toxic substances in the water. If you see that the snails become completely inactive or if the snails, especially the little ones, try to leave the water after a water change: get a product to treat the water (like aquasafe or for those with access to a lab: use EDTA or something similar).
When there are many apple snails in a tank, the water tends to become cloudy because apple snails have a large amount of microorganisms in their intestine, which help to digest the food, and which are expelled with the faeces. These micro-organisms (amoebocytes) should not cause any harm to the fish and can even serve as a food source for young fish. Fresh food (lettuce etc.) are more likely to induce this micro-organism based clouding of the water. If the snails are fed with dry fish food, the water will stay cleaner. It is a good idea to do regular water changes if you have many creatures in one tank, just like one would advise with fish, to maintain good water quality and to avoid accumulation of toxic substances and waste. By the way, apple snails are good indicators for the oxygen-level in your tank. When there isn't much oxygen in the water, the snails will regularly come to the surface to inhale fresh air through the breathing siphon. Only when there is enough oxygen for them in the water, they don't need their lung and solely depend on their gill.
Where was that article from? I need more info about the shells. I purchased a larger one, and noticed he had a crack in his shell after I brought him home. He may have some other cracks in it as well. Do I need to supplement with calcium? If so, what do I need to use?
Also, I noticed a clearish white substance floating ontop of his shell. Is this a fungus?