idk why, but my phos has gone from 0.06 - .9 with no browning while nitrates were 75PPM (hanna blinking) and everything has been fantastic in terms of color. Growth was not that bad even for acros. It'll brown if the lights intensity is low if nutrients are low. Once I raised the intensity then everything would start coloring again, but must be according to the alk level. IDK, this is just my two cents in this crazy 7 year experience. BUT I switch from GFO to brightwell phosphate-E, i'll tell you I'll NEVER go back to GFO. PS, none of my 9 tangs has been affected by it! lol
my nitrates are super high still at 75ppm and phos (my like) at .1-2ppm.
Low phos = decrease coral growth (slighttttly coral color, not much).
Higher Nitrate = increase Color
But get an ICP test as their maybe a trace out of wack that may be causing the issue. \
Hey, maybe something was changed and caused it to brown......