Half of Zoa Colony Shrinking

[muzikalmatt]

So over the past several days to maybe a week half of my largest zoa colony has shrunken polyps. The polyps aren't closed up but the heads have shrunk down considerably and they pretty much stay that way even at night while the rest of the colony's polyps close up for the night. I had been increasing the peak intensity on my lighting (a single Kessil A360WE) periodically (about 5% every week or two) and recently got to 100%. I went ahead and dialed the peak intensity back down to 90% but have not seen a change. The rest of my corals are doing fine including several other zoas/palys although they are lower in the tank. Below is a picture of the colony and my most recent water parameters. I also recently added fresh carbon (Chemi-pure Blue) and a polyfilter just in case there was something in the water, but again, the rest of my corals are happy so I would not suspect that as the issue.

Temp: 77.2 F
pH: 8.2
Nitrate: 7 ppm
Phosphate: 0.03 ppm
Salinity: 1.025
Alkalinity: 7.9
Calcium: 410
Magnesium: 1320

You can see the polys to the top and right look fine while the ones on the left and center are shrunken. I apologize for the blues. I'll try to get a picture using my polyp lab filter to give you a better idea. One thing to note is that I think the bulk of the polyps that look healthy are ones that are growing on the rock-work having expanded off of the frag disc, while most of the shrunken polyps are those still on the original frag disc.

Here is a picture from a couple of months ago when the colony was full and healthy, just to give you an idea of what they normally look like.

Any ideas as to what might be causing the shrinking or is this maybe just a temporary issue? I've tried checking at night for any pests such as spiders, nudibranchs, snails, etc and haven't seen any but I know they can be hard to spot. However, I would assume if pests are the problem the entire colony would be affected and not just a small portion. The reason I suspected it might be lighting is the polyps that appear fine might be getting a bit of shade from the rock-work above while the shrunken polyps are getting more direct exposure.

Let me know if you have any ideas or if maybe it's nothing to worry about. Thanks!

 

[reef lover]

Try moving them to different light intensities and see if it makes a difference.

 

[muzikalmatt]

A few days after I posted the "shrinkage" spread to the rest of the zoa colony and then eventually subsided.

Here is a photo from a few days ago and they're definitely looking better. It's interesting, the ones in the more direct light have longer skirts while the shaded ones have larger polyps. Now that they've grown onto the rock work I may remove the frag plate in the center section and try them in a different spot.