In this case, I don't think it is snails racing up for low oxygen. Those are Malaysian Trumpet Snails, and they're emerging from the gravel and moving upward simply because they smell the food. I have a friend who refers to them as "Zombie Snails," though they're nowhere near as bad as Nassarius snails. Anyhow....
Malaysian Trumpets are a pest snail that live mostly buried in your gravel or other substrate. They're detrivores, and don't really eat much algae (though they do eat some). When they smell the food in the water, they come out and start looking for it, though they're usually most active at night. I bet if you came down at 3-AM and turned on the lights, you'd be surprised at how many snails are in there -- you're only seeing SOME of them appear. I've seen tanks where the gravel is more snail than gravel....
You may want to reduce their numbers. The first step is to cut back on the food the snails are getting. I don't know you, and I don't know your tank, so I could be wrong, but I would suspect that the tank is getting too much food. The excess food is sinking into the gravel and turning into snail food.
Eliminating the food will help prevent the problem from going away, but it won't ever have a real effect on their population alone. You could try fish to eat the snails -- puffers and Botia type loaches work. You'll have to tell us more about the tank to get an idea of which, if any, species are appropriate. It looks like a pretty small tank, so it may very well be that none are. The better solution would be to put a trap in the tank. Take a small plastic deli container and sink it into the gravel so that the top is roughly level with the top of the gravel. Grab a small rock, and rubber band a piece of celery to it. Just after you turn the lights off, drop it into the container. In the morning, you'll have yourself a few snails! You can also use an algae wafer, some sinking pellets, or anything else that will attract the snails. A soda bottle works really well, too. Put some rocks into it to keep it on the side, and put the mouth level with the gravel. However, if you have cory cats or similar fish, they tend to get trapped, and they're a pain to get out. Keep doing it, and you'll get most of the snails.
In an aquarium, the Malaysian Snails don't really harm too much. They can add a bit to your bio load, but if you're managing their numbers, its no big deal. They may get into filters, pumps, etc and foul the impellers, causing damage. The bigger concern is what happens when they get down the drain. These snails are a major vector of disease in fishes, and a few that can infect humans, and can become massively invasive in wild systems. They can also become a HUGE problem in a sewage treatment plant; they'll destroy their filters and pumps! Man, you think shelling out (I swear that wasn't on purpose) for a new Fluval is expensive, try the filters the treatment plant is using!
