Lou from tropic Marin was saying on BRS TV that if you're phosphate is low going by the cheapest flake food you can find. The cheaper the better, they use phosphorus as a preservative and he suggested just throwing in some of that food to raise your nutrient level.
It wasn't long ago that I got back into the hobby and my Zoa's started to Brown out or just basically lose all their color. The problem was I was doing way too many water changes and my nutrients bottomed right out. I thought maybe it was my lighting and I found out down the road that my lighting was fine. I started to feed more and I let my nitrates rise around 15 and I still struggled. It wasn't until I was guided to purchase the phosphorus tester. I was very low like yourself and started dosing ab+ and started feeding more often and after time everything regained its color. It didn't happen overnight, it took about 2 months for them to adjust and be happy again.