Originally Posted by
SantaMonica
One trick to test high numbers is to dilute the water with rodi or distilled water first (add same amount), which will reduce nutrients by half and make them easier to test for.
Water changes won't do much, for long, because the nutrients are mostly in the rocks and sand, and the water change is just once. His just seems like there has been no nutrient export for many years (skimmers don't export nutrients; only food particles).
For his situation, the large system (and probably lots of rock) has a lot of phosphate stored in the rock, so will need large scrubbers and probably several of them so they can be cleaned alternately. And the high nutrients in the water will grow black scrubber growth and thus will need the strongest light possible in the scrubber so as to grow green sooner, although even with strong light it may still grow black for quite a while as nutrients come out (high nutrients grow very dark proteins).
So I would recommend what some others with big systems have done: Do a combination of big-space scrubbers (SURF4x or SURF8x), and big surface-area scrubbers (HOG3x). When the growth starts out black, it will be easier to toothbrush off of the large Green-Grabber surface of the HOG, which has more surface area than strings. And one growth turns into hair, then the SURF, which has more strings than surface, will fill in and really start pulling down nutrients.
Once the scrubbers are installed, you can see what kinds of growth starts. If completely black, then yes you can do all sorts of other filtering to temporarily help growth attach to the scrubbers. However some people start out with thick scrubber growth right away even though nutrients are high like his, and in this case you would not need to do anything extra.