Thoughts on unnexplained (and very sad) die-off

[jostafew]

Hello everyone, I'm very sad to say it but it looks like I'm about to lose the last of my 4 fish, some of which I've had for nearly two years. It all started in the summer time, I noticed that I'd lost a couple inverts (cleaner shrimp, tuxedo urchin).

After investigating I found that the SG in my mixed reef had dropped to 1.021. Long period of water changes combined with the assumption that my salt mix was the same potency as previous batches. Over a period of a couple weeks I brought that back up to 1.025 and thought I'd caught it before things got too bad. But, things continued to deteriorate. I lost the other cleaner shrimp, a brittle star, and my bigger pincushion urchin. It was around this time that I was busy and not keeping up with tank maintenance as well as I should have, and missing inverts decomposing (especially that bigger urchin) dumped a pile of nutrients into the water causing a huge algae bloom. A month or two later my Kole tang began swimming eratically, upside down, and a day later died. Very sad day. Somewhere around this time my Fire shrimp died, too. Maybe a couple weeks later my otherwise healthy and hungry Pajama Cardinal appeared dead one morning, no warning signs whatsoever. No external signs of trouble on either of them.

Through all this my treasured Crocea clam was looking strong, and I was happy about that. Fast forward to about two months ago. My clam started to decline. I'd always been concerned that it was receiving insufficient light, so I moved it from the sand-bed up the rock-work. Things stayed the same, continued to slowly decline. About a month ago I noticed that my Black clown had begun looking very thin. I'd always suspected that it had internal parasites as I would sometimes see stringy poop, but it always ate well and looked otherwise healthy. At this point I didn't have much left to lose in my reef so I dosed the system with Prazipro. Followed by a 2nd dose 6 days later. Sometime in the last couple weeks I noticed that my Azure Damsel was breathing heavily, like it would if oxygenation were a problem in the tank. Didn't see how that could be possible though considering I run the DT topless and the temperature was the same 79deg. it always was.

Finally last week during renovations in the house I tore down the old tank and began building the new. The remaining two fish, very sad looking clam, and one larger hermit crab (only remaining livestock aside from the corals) went into a temporary tank while the build happened. They stayed there for three days. During that time the Damsel died and the clown stopped eating, and the clam died.

Now, the new system has been running since Tuesday with live rock, sand, and half of its water from the old system. Livestock consists of coral, a nearly dead hermit crab, and a clown which is still not eating and I expect to die at any moment.

Along the way the only thing I found wrong in the water params were Mag and Calcium reading higher than usual, and Alk low (about 6.2dkh). I started to correct the Alk slowly, but nothing too crazy.

So, very hard to see all those creatures die under my care, especially considering that I don't know what happened... The damsel and the clown I've had for nearly two years, the Tang I'd had for about 6 months, and the pajama cardinal I had for probably two or three. It was the last thing into the tank, but it was quarantined and showed no signs of illness during QT or after being introduced to the DT. Most of the inverts were a year or two old as well. The invert die off I'm attributing to the intial low SG and a cascade effect of dieoff and poor water quality, but the fish are hardly delicate species and seem to be dying from some illness with no visible symptoms and does not respond to prazipro.

At this point the tank will stay fishless for a good two months. If there was some disease in there I want to give it time to dissipate before re-introducing any fish. I did plan to try a couple new cleaner shrimp just for some movement now that the params are stable though... Any thoughts on what might have happened and/or best way to proceed so as to avoid a recurrence?