constant algae

I have an 8 gallon tank with some Gold X-Ray Tetras and Red Minor Serpae
Tetras. I have a 125 Biowheel for filtration. The lights are on only 6
hours a day. I do water changes every 2-3 weeks. It gets constant algae.
Brighter green algae that covers the glass and (fake) plants (comes off
fairly easy). Also if I go to the 3 week mark of doing a water change, it
will start to get a different algae on the plants only, this algae is sort
of a very dark red. I have a water test kit and nitrites, nitrates,
ammonia, etc. are all at good levels according to the kit. Why such a bad
algae problem and what can I do about it?