I started mixing Pyrocat HD by acquiring the powder version packaged by the Photographers Formulary and distributed by The Imaging Warehouse, former Nova Darkroom. The Photographers Formulary version is just slightly different from the version Sandy King publicises at the PyrocatHD website (http://www.pyrocat-hd.com/html/mixing.html) because it uses 0.2 mg of Potassium Bromide for 100 ml solution while Sandy King uses half of that, i.e., 0.1 mg of Potassium Bromide.
The first time I mixed it I used demineralized water for both solutions A and B. Both came out pretty much colourless. Whenever I made a working solution it became just very slightly brown or sepia. Once I tried it using trays to make some densitometry tests and had it in the developer tray for almost 50 min. When I turned the lights on I noticed it had become slightly purple, almost lavender. I attributed it to oxidisation. After about 2 months, the small flask for solution A also started to turn lavender. I decided not to use it and instead bought some glycol to mix fresh solutions for A and B, with just the A mixed in glycol. With glycol I have not seen solution A turn lavender. All of these has made me pretty much convinced that the color change was all about oxidisation. The more so because the sepia tone is stronger with solution A mixed in glycol.
I must say that mixing PyrocatHD is both lengthy and laborious. I use an IKEA cappuccino blender to mix it and it takes several minutes to mix, specially solution B because it requires slow addition of the Potassium Carbonate. A long mixing session of solution A can lead to extended oxidisation.
I really don't think the issues reported could be due to a specific tank, unless, as others have stated, it is not properly washed.
If anyone has any idea of the differences between the Photographers Formulary version and Sandy King's version and if these are important please "chip in".