My axolotl died, why? :(

Today my second axolotl died, he was a male white albino, mature but not quite full size. I got him about two months ago and he wals living with my other, larger black male axolotl in the same 4ft long tank.
About 5 days ago I did a large water change (about 60%) and cleaned the ornaments (with cold water only) and let them sit in plastic bags for half an hour, adjusting the water untill poking holes in them so it will fill with both new and old water. They were perfectly fine as soon as I let them out, they went to the bottom, walked a bit and ate some food. both healthy, with fluffy gills. Nothing seemed to be wrong. They were very close, and always together. Yesterday he was swimming, walking and sitting fine, he still ate and wasnt showing any signs of stress. he had a slight bit of pink running through his tail.
This mornin I woke up to an upside down pure white axolotl (even the gills???) with a pin straight tail (no stress?) and quite stiff. The water seemed a bit thicker too, like mucus was coming from him or something?
My other axolotl is perfectly fine, but seems a bit lonely. He is eating and swimming, walking, breathing fine. the water is clean, ph is fine, temperature is fine, filter is very good.
What went wrong with sandkip?

That happens to alot of people I just lost an Axie for unknown reasons 2 weeks ago I have heard of 2 other people losing their animals during the same time. Hoever your animal may have died shock. A 60% water change should never be done. The change in temp, and Ph can all be stressful for fish and Axolotls. You should never really ever change more than 20%. Your tank slowly changes ph over time. Changing that large amount of water could take tour water from 6.8 to 7.8 and cause pH shock. Also taking them out of tank and putting them in plastic bags will stress them. I'm guess you hadn't done a water change in a few weeks and that is why you were doing such a large one. Try doing 20% change twice a month or 10% once a week instead of 60% once a month.

Chances are your axolotl died from poor water quality. PH isn't all that important when it comes to axolotls, as they can withstand a fairly wide variety. But other water chemistry components can be deadly. Did you test for ammonia or nitrites? How big was your tank?