Snails

About snails in general. Though the phylum of mollusks ranges from clams to cuttlefish, from barnacles to octopus and the elusive giant squid, most of the world's mollusks are snails. Though most of the snails are still marine, slugs and snails are the mollusks that conquered the land;  some of those air-breathing land snails subsequently returned to freshwater, while others never left the waters.
 
In the aquarium, good snails eat detritus, fish feces, decaying plant materials and algal films, and perhaps carrion. Bad snails add the delicate leaves of some plants to the menu. Whether snails eat fish eggs is a question I'll open in a moment.
 
Most snails lay eggs, enclosed in a gelatinous mass that often gives inexperienced fish-breeders a thrill of false hope. But the European Viviparous Snail gives birth to fully-developed miniature young, astonishing and delighting Linnaeus, who doubly named it Viviparus viviparus in his amazement. Some of our common aquarium snails like Melanoides are also viviparous. Generally, snails are hermaphrodites. They aren't self-fertilizing as a rule, so it still takes two to tangle. But any two will do. And a fertilized snail remains fertile for life.
 
Freshwater limpets. To detect one kind of perfectly harmless snail barely visible on the front pane of a planted tank you may need a 10x loupe. In the US it will likely be one of the five species of Acroloxus, and it has undoubtedly arrived unbidden on pond or greenhouse-grown plants.  Its conical uncoiled shell is so delicately  thin that the creature is seen as a whitish translucent dot, which may be as much as 8mm across. I only recognized that these minute creatures were snails by squinting through my loupe against the glass to see the rasping action of their radula,  a feature unique to molluscs. Loaches and, I think, anabantoids, search these out and make a snack of them.
 
Link. North American freshwater limpets.
 
Do snails eat fish eggs? Shell-dwelling Neolamprologus species from Lake Tanganyika certainly think so: they pick up even quite large snails and carry them as far from the empty snail shell that contains their spawn as they can. And female Apistogramma attack and harass snails that intrude near their spawning cave.  But then they'll see off almost any intruder, even an aquarist's probing finger.
 
Most snails will eat just about anything. Ramshorn snails will certainly eat fish eggs. But for me, the question is more specific: "Do Malayan Trumpet Snails eat fish spawn, especially unguarded eggs?"  I'm not sure. At Aquatic Community, where even the herbivorous fishes aren't to be  trusted with fish eggs, general consensus agrees, I'm reading that snails are recommended in the breeding tank.  At the killietalk mailing-list the question was mooted about in February 2000: you might want to follow that thread. Killifish eggs, I note, are particularly hard-shelled and resistant, but Wright Huntley's experience was that Malayan Trumpet Snails "can and will eat any fish eggs they can find," while Gunnar Asblom made a distinction between fertized eggs, left untouched by Melanoides, and infertile eggs, which were eaten.   I understand that catfish breeder Ian Fuller keeps  them in tanks where his Corydoras are breeding, in the same conviction that they will only eat dead, fungusing infertile eggs and never touch a live one.
 
Snail management. The best brief accurate account of snail management in the freshwater aquarium, Robert T. Ricketts' "Snails, snails, & more snails" from Tom Griffin's pioneering e-zine AquaSource, is archived at Badman's Tropical Fish. RTR debunked myths of snails just "appearing" in aquaria, of snails multiplying out of control, and of supposedly "safe" snailkilling chemicals (there aren't any). Robin Rhudy has a detailed page about freshwater molluscs and snails at her site, including some useful stuff, like how to tell if a snail is dead! The Apple Snail website, one of the outstanding aquatic websites,  has a section devoted to other aquatic snails. Excellent photos help you identify your snails, and there is information on snails as vectors of parasites, snails as human food, as feral pests, etc etc.