Unless your fixer bottles have crystalized material in the lid or pouring area, I doubt that it is from the fixer.
The most likely source is the tap water used for the wash. One way to find out would be to use the demin water for the wash too. You could use the Ilford fill and dump method, or else, do maybe 4 fill and dump intervals over 20 minutes then hang to dry.
The other thing to try would be a wetting agent, which is what I'd try first.