Had this flying around in my head for a few days now, and found a link that confirms my thoughts; http://www.patentgenius.com/patent/5851398.html It appears that at around a pH of 8.5 (needs to be tested), phosphate will precipitate out of solution, perhaps with some calcium. In normal tank conditions with a pH of higher than 8.5, the precipitate causes a nutrient sink somewhere in the tank. When the pH is then reduced, by bacterial production of Co2 for example, the phosphate is released once again, and the cycle continues. Be aware that the scrubber screen increases pH at the algae surface through Co2 depletion. The theory is if the water being processed by the screen has a pH slightly lower than the precipitation pH of phosphate (8.5 ish), the increase in pH on the algae due to photosynthesis will precipitate phosphate ONTO the algae cells for removal when harvesting the algae. It may be that some scrubbers require no fiddling with to facilitate this. I suggest MOST scrubbers could be improved with regards to phos removal. I have started to test this by allowing my sump to increase pH to 8.3ish. Of course this means removing my air flow to the screen. My phos level has been almost static for months now, so I should notice any difference easily.