hmm, you could literaly write a book about this, and people have, but ill try to give you a crash corse.
okay, first off, to take away a common misconception, wattage does not determine the amount of light that your corals can absorb for photosynthesis. It is the PAR rating. The higher the PAR, the deeper the intense light can travel down through the air and water and how powerful it is. Ex: NO florecent lighting does not travel down very much at all. sure you can see the light on the bottom, but it is not strong enough for a lot of photosynthesis to take place. HQI halides penetrate very very well and do not loose as much of their energy as they get deeper (although they do loose a LOT). the most commonly used rating form least amount of PAR to the greatest is probably, NO florecent, PC, VHO, T5, MH SE and then MH DE. (the last three will be debated. MH owners will always say MH is the best, t5 owners will always say t5s are better and MH owners dont know what they are talking about because they have never used them, and some people jsut like vho the best, but it realy dont penetrate as deeply as MH or t5). Now that that is covered, the amount of watts matters in each category which should be obvious. EX: 250 watt MH DE (double end) has a higher PAR (is more powerhful) than 250 watts of MH DEt. But a 150 watt DE MH has a much higher PAR than 200 watts of NO florecent. And then there are other factors to PAR ratings such as how far away they are form the corals, if there is glass seperating them, how deep they are in the tank. all these things lessen the PAR because as a photon of light travels through the air or water, it loses some of its energy. so i think that pretty much covers it for PAR
now on to spectrum. Or the "K" ratings. The only real color bulbs you will find in the aquarium trage usualy range from 5500K to 20000K. Anything below 10000K is yellow. 10000K is white, and anything above 10000K is blue with 20000K being the bluest. The higher the K, the more color your corals will have because of the blue light. it makes them florece as im sure you have seen with your actinics on. the lower the K, the quicker they will grow because the PAR is higher. im not entirely sure why, but the lower K bulbs have a higher PAR and that is all i can tell ya. lol
as far as what you can keep, once again, there are tons of variables like i mentioned above such as where the bulb is, how high up the coral is in the tank.
and i believe that awnsers the questions you posted. any others on lighting?
