What's your debris removal like? If the funk ain't getting out then something is going to be more then happy to make use of it.
A heavy cuc count can trick a person into thinking they are doing good keeping things cleaned up and pruned, but if the junk they process isn't in turn being processed/removed, then they just create a nice little nutrient cycle for you. There great at what they do but only if we are following behind them ensuring what they process gets processed. If there just free to leave there nicely formed pellets of nutrients on the sand, in the rocks, any nook and cranny then they are working far more against you then for you. And at 2 year mark it sounds as though you may have been starting to stockpile nutrients, and now stocks are full and it's giving back.
I wouldn't even bother checking tank po4 levels for now. Just keep checking the output from your gfo and change when you see the output level rise. Gha is hard to battle with gfo alone. It can get access to available nutrients pretty quickly. The gfo picks up any that slipped past not the other way around. So staying ontop of manual removal is important. A blackout could work. Never tried but read lots of success stories. Do you have any macro? A simple ats could be employed for time being to give the gha a place to grow of you choosing and in addition allowing simpler removal. Non the less, if the exports are not on par with the inputs then it's a continual wall your up against. Re examining your debris removal methods and practices would be my first coarse of action. A cube of frozen is adding allot of additional nutrients you don't need right now. Cut it back to half, and maybe every other day?
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