I don't know what kind of snails you have, but with the ones I have, which reach a MAXIMUM of about 1 to 1.5 mm in diameter, add next to nothing to the bioload.
That is another thing; many people say the 'pond snails', which typically have a long, spiral shell, are known to eat plants and get relatively large. The other common snails to be introduced accidentally(the kind I have), are like miniature ramshorn snails; they are tiny, don't reproduce too quickly, don't touch plants, are readily eaten by many fish(my kuhli's live them), and don't have massive populations living beneath the substrate.