Being up in Alaska, I know all about lack of light affecting humans. We actually process vitamin D when exposed to UV light. People in Hawaii can process and entire days worth of vitamin D in just 10 minutes or so, whereas up here in Alaska in the winter we are lucky to ever get any sun at all due to the cold (so your skin is all covered up) and the short daylight period. Most Alaskans, when given a blood test, are acutely low on vitamin D and quite a few of us supplement with pills or some even get an twice a month shot of it.
Long story short, minimal amount of exposure to light actually causes physical and social problems for human beings. People get cranky, they get bad attitudes and there is this thing called SAD (Seasonal Affective Disorder) which I described above which can really cause problems.
I'm not sure fish have anything like that, but it stands to reason that they are designed to function in the light (most fish are anyway) and therefore should be provided at least some light. You dont need to do tons, but I sure think it's a good idea to provide at least a couple hours a day.
SAD in humans can be combated by just 30 minutes in front of a 10K and decently bright light source... I would think if there is anything similar for fish, even just a little photo period would do wonders to help their mood and health.
