Snail behavior.
 

Water condition aside, what causes snails to stay at the top of the tank?

Is there a predator in the tank?

what kinds of snails are we talking about here? some kinds of snails like apple/mystery snails can stick an "arm" out of the water to breathe. if they are in a larger tank they could be staying near the top for easy access to air. just one possibility...

No predator. I think they are pond and ramshorn snails. Amm-0, nitrites-0, nitrates-10-15.

Well, Bladder snails do go floaty from time to time in a search for more food, but if they're attached to the glass, then chances are they've just found something yummy there. I know in my tanks, even if I can't SEE it, the area at the top where the water levels fluctuate with evaporation and such, tend to get a bit more of an accumulation of algae and tiny food particles. Nothing really visible to me unless I skip a tank wall scrubbing for a while, but if it's there, they'll find it. They also *tend* to like to lay eggs toward the top of the tank - though they'll lay them on any smooth surface. Though I haven't noticed pond or ramshorn snails having issues with ANYTHING (tough things they are!), many other types of snails are very sensitive to changes in nitrate. In my experience, the water in most shops tests with a higher nitrate reading, so they may just be adapting to the change in their own way. I think it's nearly impossible to kill these types of snails, so I'm pretty sure they'll be fine with a bit of time, unless you have copper or something in the water that's bugging them.

Thank you Chesherca , sounds like you nailed it as I haven't cleaned the inside glass in quite a while. Also, not all of my snails stay at the top so I now don't believe there is a problem.

Glad I could help! Maybe with these guys on board, you won't HAVE to clean the glass for a long time coming, lol! Good luck!