The pond snails that invaded my 20 gallon tank feed on green algae and
debris, as far as I can tell. The snails don't eat all of the algae off the
rocks, plants etc., but they do keep the glass immaculately clean; and they
don't bother any of my plants.

One thing to be careful of is that pond snails breed like rabbits. When I
first noticed the snails, I had been trimming my plants back every water
change (every 2 wks) so the plants were mostly small. Once the snails
started breeding, the tank looked pretty gross. There were snails on every
flat surface, more than I could count.

My solution was to first stop trimming the plants back. Once the vegetation
had grown in, the snails moved (mostly) off of the rocks and the glass and
onto the plants, where I can only see a few of them at a time. A few snails
is very pretty, where 60 or so snails visible at once was kind of
unsettling.

Second, I reduced the number of shrimp pellets I was feeding to my catfish.
I had been feeding 4 pellets daily and the snails on average got 1 or 2.
More food = more snails. I now feed only 2 pellets, and the snail breeding
rate has slowed appreciably.