Since darts are generally found around the equator, where there isn't much seasonal differenc in lighting, 12 hours on and 12 hours off is perfect. On the other hand, living in SE Idaho, as I do, in summer the sun rises at about 5:00 in the morning and may not set until 10:00 at night. I can't control this lighting. I can control their light only in winter, because I do keep them where there are windows and skylights, because they are also a part of the house decor, not a breeding hobby. It doesn't seem to affect their overall health much, but they seem to do most of the breeding activity in late fall and early spring when the natural light is perfect for them. In any event, I keep the lights as Melissa recommends on 12 and off 12, no matter what else light is coming in. For commercial breeding, I would think you'd have to have a room with only artificial light all year long if you live well above or below the equator. I think there are some tricks with misting and changing humidity in order to stimulate breeding activity, I'm not breeding deliberately, so only enjoy the show when they do.
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Patty
Pahsimeroi, Idaho
4 D. auratus blue
5 D. galactonotus pumpkin orange splash back
5 D. imitator
6 D. leucomelas
4 D. pumilio Bastimentos
4 D. fantasticus
4 P. terribilis
4 D. reticulatus
4 D. castaneoticus