What's more important - the right temp or the right water chemistry?

Hello, guys.
Sorry I haven't been on for a while.

So I have a four footer which is now equipped with a chiller and an external filter. I have four axxies in there, and a 10 gallon which has a separate axxie in, called Pig.

Now, we've had a really powerful heatwave the last few days - temperatures have been climbing into the 30s - so I thought I'd move Pig, even just temporarily, into the tank that has the chiller. I know it's too small for five axxies, but as I was going to work, I thought I couldn't possibly risk Pig getting sick from the heat - his water was 24 degrees before I floated him in to the chilled tank, which was at 16 degrees.

Now, guys, I know I've messed up. I came home to find poor Pig with his slime coat all flaking off. He doesn't look uncomfortable and actually seems quite glad of all the company of the other axxies, but he's never had an issue like this in his previous tank.

I had done a water change for both tanks that morning. On testing the water, I understood the problem. The parameters for Pig's tank are perfect - 0 ammonia, 0 nitrite, 20ppm Nitrate and 6.4 PH (usual for our area). The big tank appears to have had a crash - the ammonia was sitting at 0.50, the nitrite was at 0, the nitrate had shot up to 40 ppm and the PH was only 6!

I did an immediate 50% water change, washed the external filter out with tank water and added Quick Start. After testing again, it has dropped the ammonia to 0.25 but everything else hasn't changed.

I intend to give another big water change tomorrow, once my water has dechlorinated.

I have kept him in the big tank and moved his filter to the bigger tank to help with the cycle.

Have I done the right thing? I'm reluctant to move him back to his own tank because I know he'll get too warm. Should I move him back or keep him in the big tank?
I will of course keep doing water changes until the cycle rights itself.

I do have pictures but I'll have to wait until my husband's awake because I can't get them off of the camera yet >.<

This might sound silly but personally I would keep Pig in a separate container and float that in the big tank, then do 100% changes on his container every day. This way the big tank stays steady and Pig enjoys the cool temps.

I especially think this would be convenient so you can isolate him and keep a better eye on his condition and make sure it isn't an infection that could spread to the rest of them.

Thanks Allied! That actually makes a lot of sense!
My only concern is that the four footer is really narrow so the only container I have that will fit inside it is an ice cream tub, which is barely big enough for him to turn around in...what do you think? Should I just leave him where he is?
I'm sure it isn't an infection because it looks exactly like slime coat irritation caused by a sudden change in water chemistry and the evidence all adds up given the parameters.

Ah, I'm not super experienced with knowing all about water chemistry but I'm surprised you'd see such a change in Pig so quickly(I know you didn't really say a timeline so I could be totally wrong!). If you really feel like it is just a reaction to the water chemistry then I'd be inclined to leave them as it is and test/change the water daily as needed and see if he gets better on his own.

If not then you might try to get him to adjust slowly by keeping him separate and mixing a little water from the tank in over time (like a couple hours/ a day. However if it's a reaction to just the ammonia/nitrates (not the ph or water hardness or anything) then this probably won't help and he just might be more sensitive to ammonia than the others.

I'm not really an expert so hopefully someone else will chime in

Some axolotls are more sensitive to poor water than others, pig sounds like one of those. If it's impractical to float him in your chilled tank could you fridge him temporarily ? Would give you time to recycle the tank, your going to see more posts like yours in the uk heat wave, heat is a killer , not just the temp but the way it turns water bad faster, axys get hit with a double whammy.

Thank you for all your help, guys!

I've done another big water change today and ammonia has dropped to 0.25; nitrate is now sitting at 20ppm (although PH hasn't changed). On checking Pig today, the flaking slime coat is all but gone - there's just a few patches on his head and a little on his gills.
I think what I've done is helping!

I will do another big water change tomorrow and until the cycle is right.

Tested again today - PH is slowly creeping back up to 6.4 and Nitrate is now 5ppm. I just need to get ammonia down, which is still slightly lime green - which means it's sitting between 0 - 0.25!

Pig's flaking is entirely gone and he seems very happy now with his cooler tank and new tankmates!

I think we just had a freak PH change here, although the high ammonia and nitrate obviously didn't help. Let that be a lesson to myself - don't get complacent with tank testing! It was perfect for so long that I just stopped testing, and of course, that was when this happened. I need to stop being lazy.