In article , says...
George Pontis wrote:
I am using a Hagen/Nutrafin bubble ladder in a 37G planted tank. Lately I have
been increasing the CO2 levels to get from about 9ppm to 15-20. pH is around 7.

It seems that snails are attracted to the higher CO2 / lower pH environment. I am
finding a lot of snails around the top of the ladder, maybe 6 to 8 every morning.
I remove and dispose of all each time. There are only 1 or 2 snails visible in the
entire rest of the tank. What gives ?

Some guesses, some of them complementary to each other:
...

After watching it for a while, I should correct my original comment about the CO2
attracting the snails. That was just an assumption. With further observation now I
am leaning toward a simple explanation: there may be some uneaten food collecting
around the top of the ladder!