Who/what dunnit?

BCash74 · 2026-10-26T21:19:43-0400

Need some thoughts on the pathology here:

I awoke to this 3" cap looking like this. It was perfect the night before. It looks to me like it has been chewed on. I know I have 3 gorilla crabs in the tank (have caught 4 thus far) and many mantis shrimp - I've caught 6 thus far and still have many clicks separated by time and space (meaning I know there is more than one left in there). I apparently hit the hitch hiker jackpot with my live rock order.

Another orange cap looking great on the other side of the tank.

No other corals have any sign of distress. This evening the cap seems to have stabilized: the areas of death have "smoothed out" but have not extended. Water parameters are stable as they have been for months: salinity 1.026, ph 8.2-8.4, alkalinity 8.5, calcium 415, nitrate 7.6, and phosphate 0.05.

Only fish in the tank that could "possibly" be concerning is a magnificent foxface, but I spend a lot of time staring into my tank and I have never seen him nip at anything. I'm thinking this is a pest, but thought I'd see if anyone else had any thoughts.

Thanks in advance!

mcarroll · 2026-10-26T21:23:45-0400

I'd have to guess crabs....no other real contenders in your list. Do PO4 or NO3 get lower than the readings you posted, like maybe after a water change?

BCash74 · 2026-10-26T21:24:19-0400

Shortly after setting up the tank (say 4-6 weeks) I witnessed this event: the tank was teeming with thousands of these critters. Thoughts?

BCash74 · 2026-10-26T21:26:15-0400

BCash74 said:
Need some thoughts on the pathology here:

I awoke to this 3" cap looking like this. It was perfect the night before. It looks to me like it has been chewed on. I know I have 3 gorilla crabs in the tank (have caught 4 thus far) and many mantis shrimp - I've caught 6 thus far and still have many clicks separated by time and space (meaning I know there is more than one left in there). I apparently hit the hitch hiker jackpot with my live rock order.

Another orange cap looking great on the other side of the tank.

No other corals have any sign of distress. This evening the cap seems to have stabilized: the areas of death have "smoothed out" but have not extended. Water parameters are stable as they have been for months: salinity 1.026, ph 8.2-8.4, alkalinity 8.5, calcium 415, nitrate 7.6, and phosphate 0.05.

Only fish in the tank that could "possibly" be concerning is a magnificent foxface, but I spend a lot of time staring into my tank and I have never seen him nip at anything. I'm thinking this is a pest, but thought I'd see if anyone else had any thoughts.

Thanks in advance!

Lowest phosphate ever got was around 0.02, but that was months ago. My numbers have been pretty stable. Nitrate stays 6-9 and phosphate 0.05-0.15: I just tweak my refuge light as needed to keep them in this range.

BCash74 · 2026-10-26T21:30:06-0400

BCash74 said:
Lowest phosphate ever got was around 0.02, but that was months ago. My numbers have been pretty stable. Nitrate stays 6-9 and phosphate 0.05-0.15: I just tweak my refuge light as needed to keep them in this range.

I posted the video because part of me has been worrying that perhaps a hitch hiker of the "not reef safe" variety reproduced.... nor more likely there were eggs on the LR when I got it that eventually hatched. I'm going to have to do something significant to rid my tank of the gorilla crabs and the mantis shrimp.

mcarroll · 2026-10-26T22:43:28-0400

Looks like Mysid shrimps to me, but the lighting makes everything in the vid hard to see.

BCash74 · 2026-10-27T08:18:41-0400

The video is indeed poor quality, but the emergence of all these critters happened after midnight and all lights were out. Most of the critters were certainly free swimming copepods of some sort. But check out these:

They were fully mobile and swimming around: I’ve never seen anything like it.

Anyways, I guess the video and these bizarre critters are a distraction. Ultimately, I’ve seen gorilla crabs (at least 3 at the same time) so I know I have some work to do. To my knowledge, I hadn’t seen any damage to coral…. But it had to start sometime.

BCash74 · 2026-10-27T09:19:00-0400

Update this morning: the area of tissue loss has “smoothed out” but not expanded. Noticed one of the gorilla crabs on the rock this cap is encrusted to. My gut feel is this is likely predation by the crab and not a water parameter issue.