I am surprised that many of the corals have so much brown to them. Usually, when nutrients are too low, the zoox density within the tissue decreases so the coral becomes less brown. Same thing happens when there is too much light. One thing that is for sure arconom, your coralline is thriving.

At least it has color.:lol:
I know this hobby has gotten to the point where a lot of people are downright scared of any phosphates at all. With sps corals we want very low phosphates, but 0.00 on a high end test kit is too low, IMO. The corals need some of that phosphate to feed their zooxanthellae. Most tanks are C limited but when you truly get your PO4 down to 0.00 then the tank becomes P limited. That might be the case here. I would try feeding the tank more. Why are you only feeding every other day? I hear of people doing this a lot and I kind of feel sorry for their fish. Mine get fed heavily 2-3 times per day along with a full sheet of nori daily. BB tanks are great at allowing the user to control nutrients, so great that you can actually have too low of a nutrient level and some people don't realize that they may actually need to add extra food to compensate for this.
Most people, myself included, worry about increased algae growth with increased feedings. That is usually what happens but I dare you to find any part of any reef that has no nuisance algae. It is all part of the ecosystem. IMO, as long as it is kept to a minimum and under control, it is worth it to be able to feed a little more for the corals.
When I was having problems in my tank a few months back I worried that I was feeding too little. I was only feeding once per day, just enough for the fish to eat in about 3 minutes along with a sheet of nori. This made a lot of my corals very pale and the tissue really thinned up on a few of them. NOTE: I can't say if this was from light feeding alone as I had made a lot of changes at the time. But what I did was added another 20 chromis so I could feed more. Right now, my colors are really popping but I have made a lot of changes recently so it is impossible to narrow it down to just one.
There are a couple ways you can increase your feeding. One is by adding more fish and feeding them more. The problem with this is that if you add too many, it is difficult to remove them so it is difficult to "fine tune" feeding by feeding your fish population.
Another way is to add coral foods like GP's, rotifers, artemia, cyclop-eeze, etc. Most of these are very rich and you have to be careful not to overdo it.
Another way is to add elements that corals require more of in a low nutrient environment. The ZEOvit line offers a wide range of these but you will need $$$$$$.
What I would do, is try adding some ZEOvit AAHC. It seems that ammino acid supplementation becomes necessary in a low nutrient environment like you have in your tank and seems to play a vital role in giving the coral energy reserves to devote to producing pigmentation. This seems to be a good supplement that actually "works". I would start from there and see what happens. I would also start feeding the fish you have at least daily.