Acans mysteriously dying?

[Flatlandreefer]

I have multiple acans in my tank that have always thrived and i have never had issues keeping them. I have two colonies in particular that have been in this tank as a couple of my oldest corals, 1.5 years now. A few weeks ago one of those colonies started losing heads slowly and the dying tissue kept spreading to surrounding heads. I took the rock out in an attempt to frag it but things didn't go well and basically destroyed the colony. Fast forward a week, now the other colony is starting to show the same signs as the other colony. There are a couple heads dying while the rest of the coral seems perfectly happy. I have 6 other acan colonies and they all seem happy as can be. What in the heck is going on here???

A few facts about my tank:

This is a 2 year old 25 gallon all in one

Parameters as far as I know are in check with weekly water changes.

I did dose chemi clean a few months ago that seemed to be hard on a few corals but I can't imagine it is still having lasting effects.

 

[Braves Fan]

I would test your tank parameters ,,, you have to give the Pros some where to start ,,, with out that ,, they would just be guessing,, how big of water changes are you doing ,, what kind of lighting, what kind of flow are the corals getting ,, do you feed the acans ,,, could be a pest ,, seems unlikely for a coral that's been in the tank so long ,, what are all the critters you have in the tank ,, the acan in the picture looks healthy

 

[Flatlandreefer]

The big three parameters are all in check with alk around 9, ca around 420, and mg last I checked a week or so ago around 1450. Phos and nitrates are not tested very often but I am currently not running gfo and no skimmer so they can't be zero and I have never had a high reading.

The issue with the acan are the polyps underneath on the red colony. If you look closely the white skeleton is showing on the outside edges of the polyps while the rest of the coral is inflated. My suspicion would be a pest I guess, but what kind of pest could there be?

I have a radial file fish, Midas blenny, and clown pair all of which have been in the tank for an extended period of time.

 

[Flatlandreefer]

Lighting hasn't changed and is a kessil A360 set at a medium intensity.

 

[PlayerUnknown]

Filefish perhaps?

 

[tupes]

Can you take clear photos (with more of a daylight look and less blue) of the dying acans? Are they just losing flesh or does it look like a brown film on them? What kind of salt are you using? How often are you doing water changes? I would check the ph, phosphate and nitrates.