Dying fast!!

[lapin]

So I lowered my temperature did a water change, and then I lost my flame Angel the next morning. And I've been losing them all ever since.

Maybe pH shock, salinity shock, big change in Alk, or something in the water or container you used to do the water change. Seems like everything was fine for a month or so, then the water change and lower temp. We now know that something changed and it seem to happen with the water change. A degree or 2 temp over a few hours is not going to kill the fish. 7.5 pH will not kill the fish and many tanks run in the 7's. A big sudden change either way in pH, Alk or salinity can. Since you have LFS water, I would test your top off and salt mixed before adding it.
Next thing ; it could be a pathogen. EmdeReef kinda went thru that above and I agree that it is prob not the cause.

 

[Big G]

Two things that kill fish very rapidly are marine velvet and gram negative bacterial infections. Usually you see exterior symptoms, but not always. And yes, either could have come in with your last addition of something "wet" into the tank. Here's a link of the most common fish diseases with pictures: https://www.reef2reef.com/threads/fish-disease-index-pictorial-guide.285708/#post-3473098

 

[Kiala]

I agree with lapin. I lost a firefish due to a quick change in pH when I was new to the hobby, there was no other explanation as to why it died. My pH was consistently at about 7.8 max because I had a lid on the tank at the time and there wasn't enough 02 getting into the water. Advice was to add buffer and that ended up bringing the pH up way too quickly even when dosing the recommended amount to only change it by like 1 unit.
IMO chasing pH is more detrimental to livestock than just leaving it be and trying to come up with more natural ways of getting it up permanently (adding more oxygen) I recommend you stop buffering and make sure when you're doing wc's that temp, salinity and pH all match the tank water. Also speaking from experience here, I've had algae in my tank for months now (due to LFS water not being 0 TDS and me being stupid and not spending money on an RODI unit) I used to worry about the pH dropping significantly at night time and that having an ill effect on my fish, but I haven't lost any other fish apart from the firefish and that was ages ago so again I really wouldn't worry about a pH of around 7.8.

 

[Joe De Leo]

Does your tank water smell like rotting eggs !

 

[Joe De Leo]

It could be Hydrogen sulfide

 

[Benpastw]

How is the surface water disturbance? Do the other fish look lethargic at all? I had a similar problem that I chalked up to not getting enough oxygen into the water, placed a pump higher in the tank to agitate the surface and haven’t lost a fish since.

 

[KrisReef]

Just wondering, did you change the fishes diet recently?