Help with lps rtn (rapid tissue necrosis)

[Koralix]

I'm having an issue with my 20 gallon nano tank - my acans are spontaneously dissolving, just like SPS RTN. No signs of brown jelly or mucous, they just disintegrate from the main head. I've had this tank running for about 6 months, tore it apart and redid it 4 months ago due to hair algae. I just scrubbed the rock and glued new frags in place.

My parameters have been stable: 130-140ppm carbonate hardness, 33-35ppt salinity over the past 3 months. Calcium is at 450, up from 400 six weeks ago. Mag is 1250, a bit low, but not really. Phosphate is 0.00, and always has been for 6 months. I'm using Hanna checkers for phosphate and alkalinity, Salifert for CA and MG, refractometer and calibration solution for salinity. Heater and fan are on thermostat, stable 77-78. I do top-offs every morning and evening with RO/DI, about 3 pints a day, with a heavy skimmer. This is about as stable of a nano as I've seen.

The only recent changes are I added a sorry-looking ORA bird's nest 4 weeks ago, which is slowly recovering. I also added a mariculture milliopora frag and an aqua-cultured tricolor frag 2 weeks ago. Moved a clownfish, royal grama, and two pepermint shrimp from another tank 2 weeks ago - this is the only livestock in the tank. The milliopora frag died last week, not a surprise, wild SPS in nanos don't mix.

Now, I'm noticing some of the LPS corals aren't looking good - suddenly rotting away, no torn flesh, just shrunken heads and missing flesh, usually around the mouth of the initial polyp. New babies seem unaffected so far, but retracted. Started with my pink bobbies chalice about a week ago, half is gone, now the other half looks like it'll make it. Now three separate acan frags, main head dissolving. No slime or tearing tissue, just retracted and mouth dissolving outward.

Any ideas? I only feed about 1/2 cube mysis from a flat pack 4-5 times a week. I'd think low nutrient, but it's higher feeding than before, yet phosphates are still 0.00. The corals affected are all at least 4-6 months old, except for the new milliopora frag.

[DuskRaven7]

Sorry to hear about your situation, I've been through something similar with losing corals. I've learned to be patient the hard way - now they go through a 2-month quarantine and dips to prove they're worthy of going into my main tank. I mark the date of acquisition on the tank and keep them under observation in a bare-bottom tank. Nothing else gets added during this time. By observing for 2 months, you can catch unwanted algae, parasites like starfish or red bugs. I set up a new quarantine for each new purchase date, otherwise, you reset the clock on older purchases.

[brixo]

It's possible the initial SPS die-off released some kind of chemical that's now affecting your LPS corals, leading to their decline. I'm wondering, are you running GAC in your tank? If not, it might be a good idea to start, see if it helps stop the decline. Also, I'm a bit skeptical about that 0.000 PO4 reading on your Hanna checker - it's possible that's contributing to the decline, and you might want to look into increasing the organics in your water.