What could be causing this? Polyp bailout?

[Leon821019218]

So little then a week ago I saw that my birds nest and Monti colony started showing bald spots. With the monti it started really small with a few spots here and there. The birds nest looked like something striped the polyps, like a cable stripper can strip the mantle of electrical wire.. And it became worse every day. There was also some algae growing on the monti and when I try to remove it from the coral with a turkey baster, I even blow of some of the polyps... My water parameters are fine but I had some swings (not very bad) in Alk. Usually it's between 8.5/9.1 but there were a few spikes to 10.5 and back to 7.5 over the course of a few weeks because the dosing pump had some issues. My Calcium is 490 but that can't be it right? These corals are sitting in the same spot for over a long time and nothing has changed flow of lighting wise. I also did an (ATI) ICP test this week and the results are pretty decent in my opinion. When I tested the water against the ICP numbers it did came very close so that's a good thing I guess.. So could it be the Alk swing if there is nothing harmful in the tank pollutant wise or parameters that are completely out of whack?? I'll post the ICP test and a few pictures.

 

[MoshJosh]

Any new additions to the tank, any critters or hitchhikers that could have a taste for coral?

 

[Leon821019218]

No nothing new since a month or 2. I've been looking for any pests since it started, at daytime and at night but haven't seen anything. There is a crab living in the birds nest but he has been there since I got that coral so I don't think it's him. On the other hand, there is nothing hosting the monti's so...

 

[MoshJosh]

No nothing new since a month or 2. I've been looking for any pests since it started, at daytime and at night but haven't seen anything. There is a crab living in the birds nest but he has been there since I got that coral so I don't think it's him. On the other hand, there is nothing hosting the monti's so...

Yeah if it's living in the coral I would assume it is a symbiotic/hosting thing. . . still if you have a picture may be worth posting.

 

[Leon821019218]

I got one picture of him when I got the coral and it fell out when I wanted to dip the coral (before it went into the dip )
I was told it was symbiotic thing, the coral and the crab by the LFS.

 

[MoshJosh]

Yeah looks like what I would call an acro crab:

Trapezia Pocillopora/Acropora Crab: Saltwater Aquarium Invertebrates

 

[Leon821019218]

Yes I definitely looks like one. So that's one suspect less...

 

[MoshJosh]

So you had an ALK swing, over how many days/hours did it go from 7 to 10? I mean I don't know if thats even that big of a swing?

Maybe terrible advice. . . but when in doubt I try and do a big water change.

 

[Leon821019218]

The swings were over the course of a few days. I measured every other day at one time but my doser had issues so when I tried to correct the dosage, it wouldn't dose the amount I wanted. It is fixed now but yeah, it went on for a period of time. So it didn't spike in like a day but it did spike a few times. A water change is never bad advice I think And so I did 2 20% changes last week because I cannot hold/store more that that at one time.

 

[ScottB]

What nutrient levels are you running? Have they changed much?

 

[Leon821019218]

Nitrite is 0, nitrates are 25 at the moment (tested 10 minutes ago) phosphate 0.08. See also my ICP test it's in the pictures above. No, those don't change much. Always around the same numbers.