Populations of any life form (other than fish) within an aquarium will be governed by the available food for that species. Bacteria and snails cannot exceed the available food. So snails will reproduce but only those who can find sufficient food will live. This obviously governs fish too, but I excluded fish because other environmental factors will impact their reproduction. The same could be said of bacteria and snails, but generally speaking the environmental aspects of an aquarium are not going to be so extreme as to effectively control bacteria or snails; food will be the major factor.
The aquarist obviously provides most of the food that the fish require. Some will occur naturally in a healthy and established aquarium. This latter point is why the fry of egg laying fish do survive (provided they are able to avoid predation); a few can find sufficient natural food such as infusoria and microscopic critters, and mature fish will feed on these as well; a few eat algae. But for the most part, the aquarist provides the necessary food for fish.
Providing sufficient food for the fish without excess should always be the goal; slightly underfeeding is preferable to overfeeding. Most of us are not likely to really underfeed, but we must always resist the temptation to overfeed. Fish will appear to be hungry most all of the time; this is because in their habitat the food source may be insufficient, and if they encounter food they will eat it rather than risk starvation, since they do not know when they may again see food. This is one of many factors programmed into the fish's DNA to ensure survival of the species. And all species are programmed to eat and reproduce above all else; every aspect of their daily life is aimed at this one precept. They forage for food, avoid predators, defend territories and in some cases fry, produce hundreds of eggs/fry, readily spawn...all this is geared solely to reproduce the species.
Back to the snails...they will be present at a number for which food is available. So assuming you do not overfeed, the natural organics produced by fish and plants (this includes fish excrement, dead plant matter, algae, infusoria and microscopic live critters) will keep the snail population at the level that uses it. Obviously other factors such as natural predators (some fish that may eat snails, or predatory snails like Assasin) or water parameters (very soft water can inhibit some snail species that need the calcium for their shells) can impact this, or aquarists removing snails; but all else being equal, the snail population will be relative to the food they can find. So when I sit in front of say my 70g tank and count 700-800 snails that I can see, knowing there are many more I cannot see, I know this is the case because the fish and the system are providing them with food.
