We have 2 mystery snails in our 15G. They both came with plants. They're both pea-size, and one is roundish and always hoovering the glass and plants. The other is pointy and burrows, so we hardly ever see it.
I was watching the round snail climb around one of our plants, and I noticed the stem of the plant has a clear gelatinous glob on it with little "bubbles" in the glob. Are they snail eggs? We don't have any fish that I would expect to lay eggs in our tank.
15 gal: 1 butt-ton Malaysian trumpet snails.
20 gal: 20+ various molly and platy fry.
55 gal: 40 neon tetras.
75 gal: 2 Taiwan reef cichlids, 3 pelmatochromis nigrofasciatus juveniles.