Well your plants definitely look like they are suffering from lack of Nitrogen and Potassium. The yellowing and holes in the leaves indicated these two major nutrient issues. As stated above the snails feed on dieing & dead plant material and algae. They will be munching heavily on your plants as they fall into the mostly "dieing" category. You really need to invest in some fertilizer for your plants.
As for control of the snails - Squishing them can reduce the population but not eliminate them. You'll always miss one little guy and as soon as they are the size of a pea they've laid eggs. Same thing as trapping them, in a small tank you might eventually get them all but not before they lay some eggs.
For a tank that small I recommend squishing/trapping and also out-competing them. Nerite snails are much prettier to look at and don't lay eggs in fresh water. Adding in a few nerites and then squishing/trapping the others can eventually get rid of them.
In a planted tank these little guys are actually kind of helpful in management. If you get an explosion of snails something is off in the tank. When I first set up my 125 gallon and just had a bunch of tiny little plants in the tank, I had a massive algae and snail explosion. After 6 months and a lot of squishing when the snails got out of control, the tank balanced out and now I have maybe 50 or so snails in the whole tank. The population has been pretty stable for the past year. They reduced even further recently when I tossed in some Brigg's apple snails.
Then again I've got a whole algae eating team in the tank. 6 SAE's, An unspecified number of BN pleco's (I put in 4 but they've been breeding - I had 3 males and 1 female but I haven't seen the males in a few weeks.... the female is also looking thinner...), & now the Brigg's. I don't think I have enough algae in the tank now to keep one little oto healthy right now. I''ve been supplementing the other guy's diet with algae wafers etc. to keep them healthy.
