I have a pair of Chromis, pair of clownfish, and a lawnmower blenny in a 50. They are quite peaceful.
The blenny eats mainly film algae from the glass and rocks, and grazes constantly. He ate filamentous algae only when all the film was gone. If they get too thin, Nori can be used to supplement - people who feed seaweed for their tangs find they have enormous lawnmowers as well.
As it stands, the lawnmower does a fine job keeping itself fed, using its rasping mouth to scrape film algae clean from most surfaces.
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Alan
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