A lot of advice here, good advice, but I'm stuck on one thing. This is long, sorry, and considering the fact my tank is suffering at the moment the advice may or may not be worth the paper it's not printed on. :D I am also parroting some good advice I have been given that I think works.
I've been holding these levels for a couple weeks now. Phosphate just registered as '1' on hanna ulr.
I'm in the same boat, something went wrong, no idea what, but it's over and now comes the waiting. 2 weeks is nothing, unfortunately, it may take months. You have acros that will recover and acros that are zombies and will wither away. Keep things stable, no sudden corrections, and it will slowly come back IMO.
Some other things that stick out.
The Alks wing is probably what did the damage, I rose from 7.2 to 8.5 and it was after that when everything started to dive. PO4 was .15 so I added GFO, dropped PO4 too fast and stressed out everything that wasn't stressed out by the Alk swing.
No excuse for not having a handheld temperature correcting TDS meter and testing every time you use the RODI. $30 from
amazon, less that the cost of most SPS. You should also regularly test the output from the RO membrane before it gets to the DI filter. Ideally water leaving the membrane will be <5 TDS, preferably something like 2 TDS. The higher this number is the faster your DI will exhaust. The RO membrane is protected by a sediment filter and then the carbon filter. Both should be high quality and replaced on a regular basis. If output from the RO membrane is too high it might be damaged and need replacing. They should last a long time as long as the cheaper sediment and carbon filters are kept fresh.
IMO, get yourself some test corals so you can see what is happening with the tank now instead of trying to determine tank health with the hurt corals. Green Birdsnest is a good one, will be puffy and grow too fast when tank params are good, will die faster than an acro when Alk swings, and hates it when PO4 is too low.
My red cap is the same way. It went pale and started to die. I moved a piece to my smaller tank and it recovered (pale to deep red) within a week. Only when I gave up trying to drive PO4 down too low and did numerous big water changes (watch for KH swings if you use a high KH salt)) did both the cap and the green bird perk back up. Some acros have color coming back, some still look stressed, others are still receding and honestly will probably die.
Don't switch to a heavy feeding regime quickly, you have to go slow like with everything else. If you start to feed heavily parameters will swing and will stress out some corals. I've made this mistake as well. Stability means stability with everything. In my case my export wasn't good enough and my skimmer decided to become unstable so obviously I was swinging PO4 and nitrates all over the place while the tank got used to a heavier nutrient load. Add this to KH stress on top of starving corals and all kinds of problems can happen.
I've read where there are some people who can swing KH all over the place without their acros showing any stress. I believe it, but only if everything else is stable. This is why IMO, you can get a lot of different and seemingly inconsistent advice from folks who have experienced different outcomes in their tanks. Any single parameter out of whack may not be an issue if everything else is stable.