I'm SO sorry for your losses. This has to be very frustrating given the sound quarantine procedures you follow. The only thing I noticed in the info you provided, was the tank is about 4 months old? That is still a very new tank with a significant bioload. You added the anemone about a week before you left, gone for a week and then return to your fish not doing well. Is there any chance you had an ammonia spike? I know your tank showed 0 ammonia after you got back, but it may have spiked while you were gone? Some of the symptoms you describe sounds like ammonia toxicity. Your tank is new enough for this to be a possibility with your bioload.
I guess it's possible and I would definitely prefer that to be the answer since it means that my quarantine measures weren't for naught and circumvented through some unknown. It would also mean I don't have to go fallow.
That said.... I'm not sure. Unfortunately, I left my Seneye's slide lapse the day before we left because... well, I felt it had been stable for over a month and never shown a spike so didn't want to "waste" the slide. That said, now it definitely would have been nice to have that trend. Though I want it to be the case, I also have a hard time thinking that would be it. This bioload had been in there for over a month with nothing really even detectable from an ammonia perspective by the Seneye over that whole month with regular feedings. The only change in the bioload was the nem and I can't imagine that would do it. However, I'd never fed pellets before. I recognize they are denser food sources and can foul up water faster/easier than the frozen I typically feed, so maybe? I did have the aforementioned algae bloom when I got home. With that in mind though, the ammonia has been 0 (and the downstream metabolites have also remained 0 or, in the case of nitrate, very low) since I got back and has not even spiked given the presence of what has to be at least a few corpses which continue to decompose since I don't even know where to begin looking in the rockwork and most of it is glued together for stability. That makes me suspect the bacteria colonies are pretty robust, though perhaps even a couple fish isn't the same as a week's worth of excessive pellets? Or perhaps the colonies spiked enough with the sudden influx of food that they are now robust enough to handle that degree of decomp? I dunno, it's just all so disheartening.
I wish I had a way of knowing. I guess there's always the ability to setup a 10g tank with water from the DT and add a fish and see what happens. If it was ammonia, the fish should live, if it was velvet I would suspect it would succumb though obviously I would try to get copper in that tank ASAP. That seems cruel to me though. However, if it winds up being ammonia, it saves the clown and goby from new, long duration copper baths which obviously aren't their cup of tea either.
Ugh, I hate this. I hate killing things, I hate the tank essentially wiping so early, I hate not knowing/having an answer or clue.... as I said before, it really dampens my enthusiasm and that really sucks, because I was SO excited to finally have a tank again after over a decade.
