hob skimmers can cause that.also there could be some kind of gas exchange occuring in the rocks.if its just air though you need to find out whats causing it and fix it cause air bubbles can help hair algae to grow.
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55 gallon reef with 4x54 watt t5,29 gallon sump,red monti cap,1blue echinophyllia,trumpet,red lobo,2 toadstools 1purple and 1neon,gsp,xenia and yellow xenia,red ,many mushrooms,2 giant frilly shrooms,cabbage coral,yellow polyps,many zoos,1 orange and one neon green nepthia,duncans,cladiella |