It is alarming to think that it is expected to leave significant fixer in the film after washing - just a trace amount is supposed to be beneficial admittedly (no link, but I do recall something about that).

I have a rinse tank where I make up demineralised water and a tiny amount of Ilfotol (Ilford washaid) for each session. On the occasions when the tank has been left more than a couple of days it smells musty, though without visible growths in it. Mouldy film is not something I want so, after a day, I always use a fresh mixture. Possibly the Kodak product has some sort of anti-microbial stuff in it?