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:
1) you've got some microorganismal growth that's enhanced by the
increased CO2 concentration, and its something that they like to eat.
2) they can detect higher C02 concentrations, and their ancestors have
learned to associate that with better feeding conditions for one reason
or another
3) if it was a yeast based generator, I'd guess that they were tactic to
the smell of the yeast, with all its yummy vitamins- after all,
yeast/beer does make great snail and slug bait
4) high oxygen tensions are stressful for inverts, and they're wired to
associate the increased CO2 levels with lower O2 concentrations
5) they're wired to associate higher C02 concentrations with the higher
levels of carbonates they need to make their shells.