Any molluscide you use is likely to kill fish as well, but even if you found one that killed the snails and the fish didn't die, the water will become toxic because of the presence of so many dead bodies.
The usual means of preventing such an infestation is to quarantine plants before placing in your pond, to check for any kind of pest such as this, or '...
Garden snails eat plants, fungi and algae, depending on the type of snail, so no, I'm afraid they won't be eating any pests... you could provide food for them in the form of lettuce or dandelion leaves, if you really want to encourage them. Snails will eat rose leaves if they can reach them, but that's usually in more humid or wet regions (such as the UK) ...
Very well looks like that could be slug/snail damage. If you have a small amount of lettuce plants then just going outside at night with a flashlight and hand pulling off plants and the ground will be sufficient. Or putting a small cup full of beer will attract and subsequently drown them. This is probably all you need.
I grow a lot of lettuce and ...
A page about snail vision.
They have better eyesight than other animals in their group.
They can avoid larger obstacles.
They can't focus they eye.
Vision is not the most important requirement in terrestrial gastropods, because they are mainly nocturnal animals.
Depends what the snails are doing - if they like a particular plant, they can decimate it completely, down to the ground. They seem to particularly like young shoots on favoured plants, so if you notice leaves disappearing or being chewed, then they'll be probably be the culprits. You can either inspect regularly and remove any you see, or use something that ...
No - just treat for snails; snails and slugs are always with us, everywhere, more noticeably in damp weather, so regular applications of slug and snail repellents are usual. Snails are easier to deal with, you can pick them off by their shells anyway.
A saucer of beer will attract the snails if they are the type to come out on land at night, and they'll drown in the beer.
Snail eating fish is a good way to keep them in check, goldfish eat just about anything.