I don't know why he died

[tinyali]

My 3 year old axolotl Starbuck has just unexpectedly died and I have no idea why. I would really like to try and figure out what happened, in case there was something I did wrong to cause this.
He shares a tank with another axolotl (Helo), who is fine. First thing I did was check the water quality and it is within safe parameters (I used an API test kit).
I hadn't seen him for a couple of days, which is unusual because he usually hangs out at the front of the tank, but I wasn't really worried because there are plenty of hiding places in there. Helo often hides out for days.
Other than that, everything seemed normal. He was eating fine. I had a look at him after taking him out of the tank and didn't notice any injuries or anything.
Does anyone have any ideas what could have caused this? Or suggestions about what else I should check? I need to make sure that Helo isn't at risk.
Both of them did have a mild fungal infection a few months ago, but a few days in clean water and a tank clean out did the trick and didn't seem to change their behaviour significantly (Both a bit shy for a day or two, then back to normal).

 

[emdalliance]

The main thing I'd check is what your temperature has been like the last few weeks on your tank.

Anything above 20 degrees Celsius can slowly kill your axie.


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[Otterwoman]

I don't know what to tell you except that I'm really sorry.

 

[carsmiku]

please get a chiller. they are mandatory in my book. temp is probably the most important for these guys.

 

[tinyali]

Thanks for your comments.
I've got a thermometer in the tank, and I don't believe the water temp was too high, however I will definitely keep a very close eye on it as the weather gets warmer.
I'm pleased to report that my surviving axie is still completely fine.