I think the best fixer for these is an alkaine solution of sodium
thiosulphate - add a tablespoon of washing soda to your hypo mixture.
If you have to use Hypam try 1-2 tablespoons per litre and I would try
adding sufficient soda to make a neutral or alkaline solution. However
this is just guesswork on my part. No hypo shortage in the UK!
Peter Marshall
petermarshall@cix.compulink.co.uk