Most of the small nuisance snails come in on plants or decor, and can even come in water with fish. They lay eggs that encased in a clear gel material, hard to see unless it's on glass. If you see a clutch like this, easy to scrape off, dry out and toss. Burn to be certain they don't end up in the eco system locally.
They are not harmful, most don't eat plants but many find them a nuisance or unsightly in larger numbers. You can crush any you see and toss them. The two most common ones are bladder snails and small ramshorn snails. Bladder snails usually dark or mid brown, slightly spiral and slightly conical shell, soft shell, and very fast moving compared to other snails. Usually don't exceed /14 to 3/8 inch length. Reproduce faster than most snails. Eat detritus, some algae, keep glass clean. Most snails will eat dead fish or shrimp too.
Pond snails are very similar to bladder snails, [ all can be found in ponds, btw], but grow larger, have much larger antennae and do eat plants. Lighter in colour too and slow moving. Not sure how large they can get, haven't had many.
Ramhorns can be light brown, chocolate, leopard, red/pink or blue. Don't eat plants, do clean glass and eat detritus, thus many find them useful. Similar egg clutches, but slower to reproduce. Flat spiral shell, carried upright, rarely exceed a half inch diameter.
Another that's often seen is called MTS. Malaysian Trumpet snail. Long, narrow conical shell, depending on species can be an inch or so, up to near two inches long. Typical species about an inch though. They are very small bodied, hard shelled, and spend a lot of time in substrate, turning it over searching for food. Don't eat live plants, DO eat algae, voraciously, and many find them very useful. Have live babies, several at one time, which they release at the water surface at night as a rule. Babies are so tiny they float on the water tension 'til they find something to cling to. Often sold by hobbyists to other hobbyists, as are the coloured ramshorns.
Some loach species eat snails, and there are some predatory snails, like Assassins and Asolene spixi, which prey on other snails. Assassins are most efficient, I think, Spixis are slower to clear up a nuisance infestation.
If memory serves, assassins are live bearing. Spixis lay a clutch that clings to a plant or glass, with maybe a dozen eggs per clutch. Loosely stuck together and a bit sloppy looking, but only a few will survive to reproduce, as they prey on each other too, thus controlling their own numbers. Spixi are related to Mystery or Apple snails, smaller than Mysteries, striped dark brown on beige. Assassins are yellow and black striped.
