Apologies for the length, below is an attempt to answer the queries in the stickies:
The tank is a reef aquarium of circa 40 gallons with a sump and skimmer. It has been moved a few times as a FOWLR, but the last ong distance move occurred in Fall 2020, and sand was discarded/replaced during that move, and corals have subsequently been added a few at a time.
Background on other three recent fish deaths: In December 2021, the chalk bass went from large, fat and active to skinny, rallied for a week or two, and then passed in early Jan 2022 (fish one). Two days later a hermit crab died. In mid-February, some of the orange zoas in the tank started closing up (I posted separately on it), but after a few weeks of angst, they started to reopen, and no other corals (to include the other zoas) experienced any issues. On 5 March I added some xenia to the tank. On 12 March, I came home to find a large bangai cardinal dead (fish two), with no observed symptoms in prior days. On 20 March, I noticed the starry blenny hiding amidst the corals a bit more than usual, and on 21 March I found the blenny dead with what appeared to be a bloated abdomen (fish three). That night, one of the powerheads fell off the glass and caused quite a sandstorm in the tank -- to include burying a number of corals and causing an avalanche of sand in the clownfish's normal sleeping spot. I thankfully woke up in the middle of the night to find the sandstorm, and spent the rest of the night straightening things out, conducting a water change, and monitoring the two remaining clearly stressed fish (chromis and clownfish). The blue-green chromis reacted to the sandstorm by swimming quickly back and forth along the length of the tank for hours afterwards, but eventually calmed down and settled back into it's normal pattern of life. It's been eating and swimming normally until today, when I noticed it was struggling to swim properly. At the scheduled feeding time it swam around the food, but did not appear to eat any of it (I have an auto feeder). I also added some frozen food to the tank, and the chromis didn't attempt to eat any of it. None of the inverts appear to be affected -- a few snails are spawning, and the long spined urchin and hermit crabs appear to be fine.
I was suspicious of the first three fish dying close together, but couldn't pinpoint a common cause. I haven't changed the food. I did remove the powerhead that caused the sandstorm, but there is still decent movement and surface agitation in the tank with the remaining powerhead and the return pump. The chromis was definitely stressed from the late March sandstorm, but seemed to have recovered (I thought I was going to lose both remaining fish that night).
As I've been typing this, the chromis has started to tilt slightly back towards normal -- it's body is now positioned on a diagonal instead of straight up and down, but it isn't back to the normal horizontal orientation yet. It also appears to be starting to be able to maintain position in the tank instead of struggling to stay off the sand, but still isn't swimming "normally" yet.
I'll prep another water change and see if I can catch the chromis and move it to a QT.
Temp 77F
pH 8.0
Salinity 1.026
Ammonia 0
Nitrite 0
Nitrate 0 (usually around 5, so will retest)
Phosphorus 0.25
Copper not tested nor used
Calcium 360
dKH 5.3
Mg 1500
