Snails do contribute to a healthier biological system, but they are not much use with "problem" algae and cyano which they will not touch. But snails eat the organics (within reason, obviously they have limits) like fish excrement which breaks it down faster so the various bacteria can deal with it. I've never had nerite snails so I can't say how effective they are, but the small harmless snails like pond, bladder and Malaysian Livebearing are very useful. But again, nothing is going to deal with a problem, you can only rectify the cause and go from there.