What are you using to test the phosphate with? And your other parameters. If you have .02 then your not really stripping the water right? If you were stripping the water I would think your other corals would show signs as well. To include things being pale.
How much do you feed and how many fish?
I have only had problems with gfo or carbon after adding too much fresh media at once. If you have been running it like that for a long period of time I would be hesitant to say your water is being stripped of all nutrients. Although cutting back wouldnt hurt as an experiment of course.
Why did you start weekly waterchanges?
I would guess light and flow to be a factor as to why acros were not living.
Where were you getting your corals from that died? What were they? Wild, Mari, aquaculture?