Wow, I guess that Kodak T-Max isn't the only film that comes out purple when it is under fixed.
All 'modern' grain films, Kodak TMax 100, 400, and 3200; Ilford Delta 100, 400, and 3200; and Fuji Acros need extended time in the fixing bath.
Once in a while a roll of traditional grain film sneaks into my work flow, and I fix that for six minutes too. It's not long enough to hurt anything, and it helps me keep a consistent time for fixing film that works for everything.
Six minutes is for rapid fix (or 3+3 minutes if you do a two bath system); if you use a standard fixer that time is doubled to about 10-12 minutes.