hmmm

i am not sure if your fixer contains or has an optional hardener.
it is usually something that is added later ("part b" ) ...
it was used because older emulsions were soft and required hardening
to make them less likely to scratch &C ...

your teacher will probably know if your fixer is a hardening fixer ..
in any case ... fill soak dump and you should be OK ...

good luck!

john