Snails, even the ram's horns, will pursue blackworms in feeding glass jars or bowls. The Malaysia livebearing snail is a dedicated carnivore, That is why plant people love them for their ability to travel through the gravel, eating left over food and leaving the plants alone. However trying to raise hatch the eggs of egg layers in a tank infested with them is an exercise in futility. So I suppose in rare cases they are a threat to the fish.

However snails are also a part of that aquatic process which begins to clean up dead fish in a sometimes astonishingly short time. I think that 99.9 + % of the time, the fish were long dead and the snails were cleaning thems up.

There are marine mollusks with a toxic bite and some species actually hunt fish. But the biggest threat of freshwater snails to adult freshwater fish is as a secondary host to some disease. If you have had those snails a while, they probably aren't introducing anything new.

By the way, (this is more in response to your comments further down the page) I think our kids' introduction to the birds and the bees began with a couple of established pairs of convict cichlids and Mexican rainbow cichlids (Herotilapia multispinosa) who would spawn on cue, every storm front, while we were all home on summer vacation. (One of the perks of being a teacher was being around home more then, college courses, curriculum development and workshops not-with-standing.) A strain of golden bodied, red-tail guppies, took the explanations a little further. :)

It sure is nice having them home from college. Except now I'm reading the books they bring me. ;)