Tail Reddening looks irritated?

[Druidjuice]

My golden axolotl, toxic, has been doing fine for months but suddenly his tail started to redden. I am not sure if this is just a phase of his colors coming out since some piebald started to show on his face, but I´m still worried. Any information is welcome.

 

[Skudo09]

Have you checked the parameters for ammonia, nitrite, nitrate, PH and temperature?

Does the reddening happen more after they are active or more just consistently red?