You have about 3.5 watts per gallon. A little low, but good enough for most zoos, softies, and maybe some LPS
No, don't leave them on all the time. If you can get timers, some people like to turn on the actinics first, then the daylight bulbs. I just turn mine on at the same time... I'd leave them on for about 8-10 hours a day, turn off at night.
bulbs don't put off that much heat. I think you'll be fine. But, I would reccomend a heater actually, just to keep your temperature stable. It will drop quite a few degrees at night.
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