There are lots of predators of snails, which would eat your snails and also your guppies and maybe the plants. (Thinking of stuff like turtles, Red Ear Sunfish (Lepomis microlophus) that is also called shellcracker and on and on.) They might fancy escargot, but might also like the dieter's special (fillet of guppy).

Some of the aquarium loaches from Asia will be pretty effective in eating snails and should leave your guppies alone. Don't know how guppy fry will fair though.

Scott Lockwood has pointed out that the relatively small skunk Botia (formerly Botia morleti or Botia horae and now Yasuhikotakia morleti) is surprisingly territorial and something of a bad actor. I don't know that I would risk a guppy's tail around them. :)

http://www.loaches.com/ is a pretty good place to look for info on loaches. The scientific name game is more than a little confusing. The first published name used in a scientific description is considered the proper scientific name except in really rare situations where the first name has never been used and another is widely used in all scientific circles. Looking from the outside in, I'm guessing that westerners described several loaches and now it is being discovered that Asian scientists had described those same species before the French/ British/German workers did. That is the priority rule.

As people become fluent in different languages, as books and publications are more widely distributed and as information is more available everywhere from universities to on the Net, researchers are discovering these duplicate names. This has been a century long and (for hobbyists) uncomfortable process, unless we treat it somewhat like a sport or soap-opera. :)

One drawback to using loaches to get rid of snails is that we don't want the biological mass of the snails taxing the tank's capacity to process waste materials. But the loach or loaches will weigh more and tax the system even more. Two ways around that:

1.Put in some old lettuces for a bit. When several snails have attached themselves to the lettuce, take it outside, shake them off on the sidewalk outside and smoosh them so they don't suffer long, while suffocating.

2.Get some fish or a fish (snail-eating cichlids or sunfish get large and chew up the snails with "pharyngeal teeth" in their throats) that eat snails. I keep several darters in the living room. When a snail gets dropped in, they move it around almost like a soccer or lacrosse ball. ("Darter Football") Although I haven't seen the actual process, they must suck out or (with their tweezer-like teeth) pry the snail out of the shell. Darters will only eat live foods (ka-ching!) or freshly defrosted and rinsed frozen foods, which move in the current. I'm so grateful for extra snails to put in that tank I have to be careful to not over-harvest. :0

There is an irony associated with water chemistry. In the livebearer tanks and in aquariums with hardwater North American killies and minnows there is an abundance of snails. The water encourages them and they must be able to absorb some calcium from what they drink. Alkaline water also encourages the growth of algae and the snails feed on or in that too.

In tanks where I'm trying to breed rainforest killies, some Corys, tetras (if I did) or even a few soft-water North Americans (dwarf and pygmy sunfish, Fundulus cingulatus. Leptolucania ommata...) demineralized water is added to the tank until the mineral level is less and the fish (happy fish are spawning fish) are reproducing.

BUT the water is relatively low in both the minerals, which would be used to build snail shells and to buffer the water to a higher (alkaline) pH. So the snail shells dissolve some (you will see a whitish spot, discoloration or even holes in the shells). At the same time it is harder for the snails to replace those minerals from the water and I have had several snail-free tanks. Then, if scavengers are needed with fry, snails must be taken from the hardwater tanks and placed with the fry (for a time anyway.)

If your snails all start dying off in a guppy tank, it is time to start doing more significant partial water changes. Perhaps because of the growing guppy population and the increased amount of food they are processing, the tank's pH is dropping as the beneficial bacteria process all of that "stuff". That is not good for the guppies beyond a certain point and disaster can beckon if those changes aren't made.

Hornwort and Val are hardwater plants and great for guppy tanks. It has even been suggested that they help buffer the water and pH up. I don't know how they would do that. But if they get "mushy" (another precise, scientific term) check the water quality and pH or just get going with water changes or even re-setting up the aquarium.

By the way, in response to a question on a (long gone but missed) livebearer's list as to are there soft-water livebearers, Harro Hieronimus mentioned a couple of the Micropoecilia from Brazil and the Guiana Shield. If I ever get a pair of picta or the like, then I will worry about their water.

By the way, Harro's very excellent "Guppies, Mollies, and Platys (Complete Pet Owner's Manual)" in paperback seems to have been re-issued in a new addition in 2007. That inexpensive book is a keeper for livebearer enthusiasts, especially those of us only in the hobby a few years.

If you have small children, snail picking can become a game. Turn off and unplug the light,  unplug the heater, and other accessories! If the filter or air pump is some distance from where they are working, maybe it can be left on. Remember to plug everything in later. :)