Mass fish death overnight - except clowns

[Jay Hemdal Verified Community Expert]

This will be my second-to-last post on this thread so we can move on.

My last post will be a week from today, for anyone who might still be interested in this and for any potential future viewers who may have stumbled on this thread. I’ll provide a wrap-up summary of what I’ve learned and any future tips I may have given to myself should I have known about risks associated with the most plausible drivers behind the die off.

It is now around 50 hours after the initial discovery of the die off, and the remaining fish are still behaving normally, which I define as no labored breathing, no erratic swimming patterns nor listless swimming patterns, no swimming against return pumps or power heads, demonstrating interest in appetite.

The inverts are also alive and, incidentally, two of my three shrimp have molted yesterday.

That all pretty much rules out anything except a transient water quality issue, of which oxygen demand and/or high CO2 are the two common issues. I think turning off the skimmer and adding bacteria was just enough to push a percentage of the fish over.
Jay

 

[keizo]

Just to wrap things up here - all the remaining fish are still alive, along with my inverts and corals.

No notable signs of marine velvet disease (the primary suspext), but my yellow corris wrasse did have a strange incident yesterday where he came out with slightly red underfin markings. I thought to myself: this is it. It was uronema. He was breathing with labor on the sand bed, both sides under his fins sporting that almost bloody or bruised-like look. He was even upside down on the sandbed at some point and I thought he was a goner and I finally had a lead as far as what caused last weeks incident.

But I checked back no later than 1 hour after the initial sighting (he had found himself a hiding spot between rocks I couldn’t see) and the redness disappeared, he no longer was hanging on the bottom of the sandbed, and was breathing fine. I have no idea what to make of this other than potentially some bruising or unknown neurological wrasse disease symptom but I checked again this morning and he seems normal again.

if this becomes something I’d like to monitor further I might open up a new thread. Until then thanks everyone for your help!