Over a year ago I broke down my 120. When the tank was setup I had a hair algae outbreak. The rocks have been dry for over a year. I pulled,brushed and picked off the dry hairalgae the best I could. Then I saw a post here on frag on how to acid wash rock. I followed the directions given to me. I now set up my 40 gallon aquarium with 1 metal halide 250 watts. I noticed this morning that I have algae in spots on different rocks. I have been out of the hobby for close to 2 years now. Is this algae from the light or did the acid wash not kill the old hairalgae? Oh I am 3 weeks into my 6 week cycle. Next week I do my first water change. I also have put in 1 pinch of flake food twice in the three weeks that the tank has been cycling.
when you dried and acid washed your rock it killed any living organisms inside the porous cavities inside the rock, essentially trapping dead organic material and hence phosphates
now that you are restarting with light and sea water, those phosphates provide adequate food for algae to grow
ideally, it would have been good to leach out the phosphates using lanthanum chloride to precipitate phosphates in a filter sock, prior to setting up your tank
however you can still do now, if you haven't added livestock yet
run a small pump, run tank water through a filter sock and simultaneously drip lanthanum chloride into the filter sock, it will bind phosphates and then clog the filter sock, keep cleaning/replacing the filter sock until it no longer clogs
you can pick up a bottle of lanthanum chloride at any pool supply store, its used to lower swimming pool phosphates, you should dilute it 30:1
