Can’t speak for Zoas, but I’ll share my recent experience since you mentioned the hammer and I feel like it may be relevant overall. Summary at end b/c I feel it’ll be somewhat long but wanted to be thorough to paint a better picture.
Basically had a neglected tank after a velvet/ich wipeout. Just kept my snails and shrimp alive for 8 months? lol.
Decided to get back in it early Feb. Amongst fish, got 3 hammer coral frags. All opened as much as I saw at LFS and seemed healthy. No nutrient testing yet but I imagine it would be high-ish (lots of algae from the neglect) and ghost fed before to keep bacteria and critters alive.
Then I went on a cleaning rampage. I vacuumed all the sand (nasty) repeatedly, 5%-10% water change everyday while cleaning sand/rocks for like 2 weeks, scrubbed all the algae off.... refug and skimmer running. Tank looked pristine. Then Cyano came, then with it some brown stuff that may or may not have been Dinos (thought it was detritus stuck on bacteria bloom initially from the sand cleaning). And while 2 hammers were still ok, one (the bigger with 4 heads) closed up to where you can see the skeleton “teeth” throughout the day.
side note: Also had 3 rbta that initially were open “fully” but seemed to close to 3/4-ish? Also a red planet acro and random acro frag rtn few days apart. 3 other frags ok.
Did some searching on forums and it seems now a common problem is too little nutrients (no3 + po4), both for coral health and/or Dino issues (and maybe cyano).
So past couple weeks, reversed course. Fed fish more, added amino acids daily (acropower) for nitrates, added reef roids daily for po4. Note: started the doses small-ish b/c no test kit for nitrate/phos yet..). Stopped the water changes (except replacing some water for siphoning cyano/brown stuff with feeder).
Within a week, the 4-head hammer fully opened (maybe more than before), the other 2 hammers seemed to be open more, the rbtas opened up even more than when I first got them, no sps deaths. Cyano and brown stuff/dino has significantly reduced (cyano still working on but again, much better).
Got the test kits last week and initially showed 2.5-5 no3 (Salifert) and .21 po4 (Hanna). I wanted to sustain that or maybe raise no3 a bit/lower po4 a bit so continued amino acid dose, cut off reef roids. Fed fish about the same. Testing everyday, my no3 has peaked around 10-25, po4 kept rising (guessing old stuff still breaking down?) to .31, but yesterday test .27 so maybe trending down (even considering .04 accuracy error, at least not going up).
Hammers still happy and more full, rbta happy and more full, sps frags seem happy (polyps out, colors good and one milli that initially lost some color coming back?).
SUMMARY
tl;dr - bought hammer corals, placed in what I assume a good amount of no3/po4 water and they seemed well. Went on cleaning/water change rampage which I assume lowered my nitrates/phosphates very low, very fast (based on what my no3/po4 tested for even after heavy fish feeding/amino acid/reef roids). One hammer in particular closed up a lot. Got cyano and some brown algae or dinos.
Raised no3 to 2.5-5 (Salifert) and po4 to .21 (Hanna), hammer open back up (more than before). Other corals and rbta happier. Cyano and brown stuff/Dino reduced significantly. Now at 10-25 no3 and .27ish po4 and still healthy. The specific numbers aren’t important, just more for reference.
Lowering nitrates and/or phosphates too fast, or almost stripping them to undetectable, in my experience (although limited and recent, and somewhat based on assumptions since didn’t test early) and based on forum reading (which again, can be all over the place so gotta parse it and make own choices with eye test on your own tank I think lol) can lead to poorer coral health and/or other issues. I mention this because although you said Sunday your no3 was 5 ppm and po4 .3, you mentioned now it is 0 no3, .03 po4, which seems like a big swing (esp po4 side?) and undetectable nitrates might be issue? (Dont know if the zoa/hammer issue happened before or after Sunday).
