I think a clearing time of only 90 sec is not realistic for all types of emulsions, I think Deltas and TMax need more time. The normal fixing time is two times of the clearing time for normal emulsions and three times for Delta and TMax with structure cristals.
You take a small piece of film from the end looking out of the cartridge (for 35 mm) and place one drop of fixer on the emulsion. Then wait until it gets nearly transparent. Then dip the film into a small glass with fixer and agitate it a little bit. Take the time until you cannot see a difference beween the first clear spot and the rest of the film any longer. Place a white piece of paper with text under the glass so you can see better the transparency. With this method you can check the exhaustion of you fixer and the time for an unknown film.