Tom,
Aquaman is right that the answer to pollution is dilution along with a reduction in feeding. 3 cubes a day is a lot (IMO) for a 90g along with the rotofest that will surely drive up the PO4 and NO3. If it were me I would attack high PO4 with reduced feeding and a regiment of water changes. In the past when things start to get higher Ill do a 20% WC then follow up every 2-3 days later with 10% WCs for 2 weeks then tapper off with a maintenance of biweekly 10%WCs.
Also, what type of skimmer do you have? and if I remember correctly you have a fuge or algea scrubber...I think that your problem stems mostly from your food and additives than from inefficent nutrient exports...youre just putting in too much waste.
And just to chime in here, I too used to run carbon 24-7 but within the last year or so have dialed it back to a day or 2 every 2 weeks after a waterchange. I feel like I have had better results employing carbon in this fashion than before. I never liked it in a reactor (over polished the water IMO) and I tend to agree with others that it starts to leech back into the water column after a while. about 1 cup rinsed in a filter bag near my drain pipe is all I use now. And you must rinse the carbon extremely well in a mesh collendar to clean off any debris/powder.