I had 3 nassarius snails in my old tank and gave them away when I switched back to a nano without a substrate. They didn't do much of anything visually in my tank, they left my sand bed undisturbed and only came out when I fed my fish. They would then glide right over any algae without touching it and go for the food that reached the bottom before a fish got to it. I'm sure they moved the bottom layers of sand around which is good but they never touched the surface of the sand bed. If you're looking for something to clean up the visible area of your substrate I don't think you'll be happy with just these snails, if you want to prevent lower levels of sand from becoming anoxic then they have a bonafide purpose. my 2 cents.
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