I dont know if you mean by converting to electricity, then useing electricity to power conventional lamps, or if you mean useng natural sun light to light the tanks, but I will give you some tips on both.
Conventional tanks are hugely power hungry beasts, and to power them with photocells would be VERY VERY expensive, and require possibily all of your roof space depending on what the weather in your area is like(if its often cloudy, it could require many times more cells or roof area than say arizona.)
Now, if you are purposeing useing natural sun light, than sir, you are definately onto something. I plan to do this one day when I have a real house, not just a damn apartment that limits me from doing so many fun projects.
Reguardless of where you live, you can definately light your tank with NSL (natural sun light). Lets say you live in the most cloudy part of canada/washington/whatever. You dont have nearly the amount of light energy per unit area that fussy corals demand. However, this doenst matter at all, thanks to the ability to easily concentrate NSL from a large area into a small area. For example, lets say your tank is 4ftx2ft, thats 8ft^2 of area. Lets say on your most cloudy day you only get 1/8th the light intensity that a tropical reef sees. No problem, harvest light from an 8ftx8ft area on your roof (or whatever), and use perhaps 4 freznel lens to focus this energy into 4 solar tubes, which would then have a direct path down to the area above the aquarium. I have read that if you use the included piece of polycarbonate used to block UV thats included with the solar tubes that you just grow a bunch of algae and corals brown and die, so make sure you dont do that.
Anyways, now you have a problem, what happens on really sunny days? Your corals would get 8 times(ehh, roughly) the light and fry. Well, there is an easy solution. Use a small solar cell on the roof with a small load on it to determine the current NSL intensity. Use this input signal to control a simple stepper motor or even RC toy servo to actuate anything that can mechanically attenuate the light intensity going through the tubes. Something as simple as a chimney flu could do the job easily.
Anyways, for night you could have them just all stay wide open and your corals would get proper lunar light cycles, along with proper daytime seasonal cycles with REAL sunlight. Now, being so far north, your photoperiod is going to be shorter than the corals would get in nature, but due to corals wide range of adaptability with reguards to photoperiods, I think this would be of fairly small concern.
If you decide you wana do something like this, let me know and I can build you the light attenuation circut to control steppers or servos for about $10.
