Hammer corals are receeding. Water parameters are fine

[clown99]

After 2.5 years of growing from 1 head to 14 heads + dropping new ones, my hammer corals are now receeding. The smaller one with 4 heads is already showing skeleton. It started about 3 weeks ago. Water parameters:
Temp: 77.7
pH: 8.38
Salinity: 1.024
NO3: 0 (Salifert)
Alkalinity: 9.3 (Salifert & Hannah)
Ca: 460 (Salifert)
Mg: 1200 (Salifert)
PO4: 0.05 (Hannah)
Tank: 29g Biocube, no skimmer, Chaeto with grow light in chamber 2, UV light in chamber 1.
I don't have a clue why this is happening. Has anyone experienced this?
Any idea what I should do? Thanks in advance.

 

[HB AL]

Add any other eyphyllia in the time its grown into a colony? If you do an iodine dip or such check to see if any flatworms come out. If its not pests then I would look closer at all water params and send out an icp test.

 

[monkeyCmonkeyDo]

+1 have u added anything to the tank before this started?
D

 

[Beau_B]

The bottomed out NO3 could be an issue... starving.

Change to flow?

 

[clown99]

Nothing has changed in terms of flow, or new things added to tank because of covid19.
Since I took out the protein skimmer and added the grow light to the chaeto, it is growing like crazy. And NO3 became undetectable, even though I added ESV Nitrate 0.5mL/day 4 days prior to testing. Maybe I'll cut back on the chaeto light and put back the skimmer. Maybe it is starving for NO3.