I have an infestation of snails that came in on some live plants. At first I thought they'd be helpful and keep the tank clean but they aren't doing the best job and since they have been in the tank I have gotten a butterfly pleco to clean. So I need a humane way of getting rid of the snails, it doesn't need to be fast just effective.
No fish "clean" the tank. "Butterfly Plecs" are actually hillstream loaches and have very specialist requirements. They need well-oxygenated, clean, cool, fast-flowing water, and are therefore unsuitable tankmates for guppies (the long tails makes them poor swimmers, and they won't like the cold temperatures the loaches need), and a 35L tank is too small for them. They feed on the various micro-organisms that live in algae, and to do best will need to be in a tank with a supply of algae covered pebbles (you may need to set up a "pet rock production line" on a windowsill to supply them with the food they need).
If you want something to get rid of the algae on the tank glass you're better off either doing it yourself with an algae magnet, or the "razorblade on a stick" that Algarde do. If you must have an animal to do it for you, then Nerite Snails would be a much better option in a small tank.