[jenglish]

More than likely not a true algae but cyanobacteria, a photosynthetic bacteria. It grows in the presence of PO4, low flow, or old lights. If your lights are past their prime replacing them may help. Increased flow can make it harder for the cyano to form. You may not get readable PO4 on a test kit because all of the PO4 is bound up in cyano. A 3 day blackout can free up the nutrients for a large water change to remove them. After it has been knocked back regular nutreint management such as wetter skimming, large waterchanges or cut back feedings may be in order. Good luck

 

[jenglish]

if you want elaborate search "rock lifts" on this site. If you want simple I have seen PVC pipe cut to only be a 1/4 inch taller than the sand used instead of base rock.