Hoeben (petstore with a nice seewater part) advised against algae to check KH and move it up to ca. 12. My check today (last column):
Test Min Max 3-11-15 24-12 12-1-16
PO4 (mg/l) 0 0,03 0,03 0,05 0,03
Mg (ppm) 1200 1500 1200 1050 1170
Ca (ppm) 400 450 380 440 425
NO3 (ppm) 0 5 50 25 5
KH (dKH) 8 9 11,8 13,3 12,8
pH 7,8 8,4
So KH is already on the high side. I found explanation about KH at http://www.zeewaterforum.info/forums/archive/index.php/t-89310.html?s=02f0c1feb0687278ce701d21e0a72c1d most useful.
I bought DinoX and applied this toxin also 3 times so far (like DinoXal last month), but with no result against the filament algae. After 3rd time I saw the skimmer residue turned a bit greener, so maybe have to continue a while.
Meantime refreshed 40 l (35%) of the water (hence the nice values above;-). I also took out the major rocks and brushed them clean.