I don’t really like using labels like ‘good’ and ‘bad’, but they have the advantage of being short, concise, and not-too-inaccurate. Good Snails are the ones that a) you pay money for, b) you give names to, c) you care for, and/or d) that you’ve intentionally raised from babies.
Bad Snails are the pests that sneak in on plants and things, that reproduce like rabbits on fertility drugs, eat all the food, clog up the filter, and generally have all the appeal of fleas on a dog. These are the ones that you spot one or two, then the following week there are four dozen, then the following week there are eleventy million.
They actually pick on the Good Snails, clinging to them like the aforementioned fleas, picking at any faults or flaws in their shells, laying their eggs on them, and driving the Good Snails, and myself, quite mad with frustration.
In the past 6 months, I’ve encountered several species of pest snail. There are the Physidae, commonly known as bladder snails, tadpole snails, or occasionally pond snails. These ones are sort of foot-ball shaped and have sinestral (left-handed) shells, with no operculum (no shell door). The Physidae breed quite quickly, but have thin shells that are easily squished or cracked, and many fish will eat them.