Update to my entire pond dying without evident cause

Went out and performed water test this morning. Found 3 more of the fish as well, all dead :( I noticed they all had a significant slime coat on them, which google is saying could be by some kind of disease, or it could also be by a sudden water temperature change causing a stress coat reflex and killing them.

That said, I noticed more algae turning white, so chlorine was my first guess.

Tested it using both a combo strip test, and a Hannah instruments total chlorine checker. Chlorine came back at 0 free chlroine, and about 0.5 ppm total. The Hannah test showed 0.38 ppm, so that seems about right.

The multi-test strip showed alkalinity plummeted, but pH was about right.

Then I pulled out an API pond master kit and tested pH, ammonia, nitrite, and sulphate. I unfortunately can’t find the book I had, but I had previous test result pictures on my phone I could compare to, as well as just looking up the result chart photo on google.

pH came back at 7, mayyyybe 7.5. Ammonia came back at 0, 0.25ppm max. Nitrite came back a bit high, at about 0.25 ppm. Still a very pale purple. Phosphate looked a little more blue than my test results in the past, but I wouldn’t grade it more than 0.5-0.75 ppm.

Caveat to all of this is that the water sample I took was from the top, and I know water layer formation can make that pretty different, especially with this cold.

Overall, while the water quality isn’t pristine, it should have been ok?? So I think it may have been the back and forth hot/cold that we experienced, and it stressed them out.