I have used some Tetenal stuff, and the keeping quality's are very good, I am still using fixer from a 5 litre bottle that I bought 3 years ago, and it is as good as the day I bought it Tetenal is one of the leading chemical manufactores in Europe, making stuff for certainly Patterson, and I believe make some of Ilford's chemistry, so they must be pretty good, I have used their toners, and at times their print developers and never had any problems. I have also used Indicet stop bath and it lasts ages, never goes off also their Mirosol 2000 is very good, lasts forever, so I wouldn't worry about them
Richard