black algae

from 8.2 to 8.6 is a considerable difference and is your 8.2 with lights on, ph climbs higher with lights on and falls when out, kalkwasser will hold the ph even with lights out, this will slow algae growth, deep small grain sand beds do not get the water penatration you need and after awhile can crash a tank in my opinion 2mm or more grain size should be used for deep sand beds to work the way they should. but everybody to there own, has to learn what way they wish to go. you say you have been doing water changes and the algae keeps coming back so water changes do not stop a bloom. i'm trying to give you another way to get rid of the bloom these things have worked for me, the lights must have the correct color temp to keep algae down also old bulbs can cause blooms i hope this helps

 

That's will certainly do it. You can put GFO in a filterbag and put it in you HOB. However, .25 is 50x higher than the ocean, that's a ton of phosphate. Although, you are probably using a cheap inaccurate test kit, but getting any reading on a cheap test kit is a bad sign. Definitely think you'll need to run GAC, GFO and do frequent water changes. Sihon whats there. Get your N and P to undetectable levels and go from there.

 

Is it just growing on that rock? Then I would scrub that rock in ro/di water then place it back in the tank. Couldn't hurt anymore than you did with the salt water scrub.