Bettas like having a day-night cycle, but they don't like really bright light. They are low-light fish, but still diurnal.
My Betta Caspian let me know all about that. He's inactive when there is no light, so he wants some sort of light. However, if I put a really bright light on for growing my plants he will start tail biting and hiding in his cave for shadow. He will also stop building bubble nests if the light is too bright. I'm experimented with this a lot, and have found that he likes lighting if it at or below 2 watts per gallon, but above that his tail starts disappearing again...and he becomes washed out, hides, and is inactive.
While he is obviously overreacting to it with the resort to tail biting, I think that the light preference applies to most Bettas.
Last edited by Akeath; 11-08-2011 at 06:08 PM.