how would algae not be an issue?
If you export nutrients, have good husbandry, run GFO, and carbon dose, you will have little phosphate for the algae to feed on.
I was under the impression that algae could not grow without PO4, but the quick research I have done says it can as some of you stated. Apparently no PO4 just slows the rate of growth dramatically.
I would see no issue with a 15 hr photo-period when disregarding algae. I have to ask though, why can't you feed your fish twice within the normal 8 hr photo-period?
Because I leave my house at 6a.m. and don't return until 8 or 10 at night during the week. I am awake in the house for basically three hours during the week...an hour in the morning and two hours at night.