The fixer concentration is not critical, so a small amount of undissolved chemistry is OK and can be filtered out if necessary. Alternatively you can heat the solution to a higher temperature to try and dissolve more of the residue but obviously the heating will need to be done in a non melting container such as stainless steel or pyrex glass.

A hotplate with a stirrer bar is very handy for mixing.

A comment on dry powder chemical dust. If you are a careful worker and take precautions in the way of minimizing dry powder dust in the air you can use the same space for drying negatives as for mixing powder. But it is clearly not the best situation for minimizing the risk of negative contamination. A well exhausted space is a far better option when possible. In my early bathroom setup I used a cardboard box with an exhaust fan and dryer hose vented thru the bathroom window.

Nate Potter, Austin TX., Chatham MA.