New axolots after my two beloved cuties passed away

Hi, in another thread I described the awful two last weeks of my second axolotl, P!nkie. i had two axolots who now sadly passed away. One was nice and plump, ate until the day before his death, had no symptoms whatsoever. The water had been too warm for them for a little while during our too hot summer this year, so I guess, that, even with daily waterchanges and icebottles, it was too warm for him, altho i did not see any symptoms.
The other was, as the doctor described, unwell since I got him (as a baby) as he has always been on the small and skinny side. The vets cannot rule out that the illness he had for the past two weeks was a fungal infection, but he does not know certainly.
While my little P!nkie was at the vets i decided to buy mew stuff for the tank to male it nice for when she comes back... Which she sadly never didi
I bought white sand as on the darker sand I can't see the poop very well and after her problems I want to be able to monotor that and to see straight away if there's some food left or not...


Anyway, I am heartbroken at the moment, but I don't want to be without axolotls in the future. So I wanted to ask you, as that last axolotl was ill, it might have contaminated everything. I plan to throw the whole sand away and to replace it with white sand. I will replace the fournitures as well. Now my question for you would be: is it besser to take the entire water out and to desinfect the tank (which products to do that?) or shall i keep doing regular waterchanges (how much and how often?)? What about the filter? Does it jeed to be cleaned? Not cleaned? Replaced? i'm not sure as on one hand i might have something nasty in the water, on the other hand the tank has been running for four years and is fully cycled. Shall i cycle it again, from the beginning? Or is it better to keep it cycles how it was?
I have time, as i don't want another axolotl before: i have salt for salt baths, stuff for tea baths, understood that thing with that hoffstaedter solution or how it os called, found a vet that has treated axolotls before, know where to buy waxworms...
In the meantime, I would appreciate your help so that, when, after grieving as well, I will get new axolots to start as good as possible. Apparently ot was jot my fault that my axolots died but i still feel responsible and want to try to make it even better.
Thank you so much

Hi, thank you
I guess I will worry about everything. But I would ask what people in the forum think first as I am no expert. But I think if axolotl have something on the weekend that could be treated with salt or so and i dont have any around and wait to long it could be too late. The thing is, i am sure the vet i went to (as his name was the only one on a list of herp vets where I live) did his best to try to save him but when I was there at first he thought what i saw as red spots were just his veins that were slightly red and that the problem was internal. This could of course be true, but the problem is, i dont think any vet here has ever treated an axolotl so i guess i would have to be a reliable help for the next axies.
What do you think about the filter? Keep it as it is with all the bacteria or rince ot?

That was what I was thinking but them I was not sure if the waterquality would be better in a longer esthablished tank or in a newly cycled one...
But yeah as it cannot be excluded that it wasn't something contagious I'd rather start again to be fair.
The fishlessmethod as opposed to?