Henry, I have used a lot of the tetenal superfix plus, it is available at very good prices and I get it in 5 litre batches and I have never had any problems, it is as good as the Ilford product but better priced, however, I would suggest you use it at higher dilutions for film fixing, 1/4works for me, 3 minutes for normal films, and 6 for Tmax films,
Richard