Sometimes that stuff gets a foothold in your tank and it seems you battle it forever. A couple of things that I might try if you can is to turkey baste the bejeezus out of your rock - even every couple of days and see what your skimmer can pull out. I would also be tempted to do a 50% water change after basting. If you can pull out the rocks that are the worst and take a scrub brush to them in a bucket of change water. I would also cut back on lighting to say 5-6 hours/day on MH. I know I have cut mine back for the summer because of heat and also because of the amount of ambient light there is in the house. Also try cutting out one feeding a day if you can - not sure what critters you have so don't know if this is possible.
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