replacing T5 bulbs

[Fishward]

hey,
i bought a used fixture last year and have been using it for about 6 months. it ran for 2-3 months before i bought it. im noticing slight changes in the light color. should i be replacing all the bulbs? or do actinic bulbs last longer? ive got 3 daylight bulbs, 2 Actinic, and a fiji purple... do they all need replacing every 6 months?