OK they are both sort of right and i will try to explain a few things to help make it clearer but in all honesty i will probably make it just complicated
Actinics are a very cold temperature at roughly 24k which is very blue. They are used primarily in the marine world as a dusk/dawn period before the main lights go on and to run alongside the main lights during the day also. These are softer lights and its usually a great light period for fish to adjust to going from a night to day cycle without being plunged into either suddenly.
Corals that like a lower light lvel will gain "some" benefit from these early morning and evening periods but in all honesty the light that actinics give off does very little to aid the corals and is usually for asthetics to be honest. (They do make colours very vibrant)
Blue is the very last colour to be absorbed by the sea so its also the deepest penetrating light you can have in a tank. In theory, corals from deep water would benefit more from the blue light but deep water coral are very hard to keep.Corals that live near the surface like acros etc want the higher temperatures that the sun can provide (if acros liked blue light they would be found lower down in the sea) so actinics give very little benefit to them at all.
Natural sunight has a temperature of 6.5k (i think
) and if you had this over your tank then your corals would thrive! The trouble is the tank would look like cats wee
A comprimise is usually sought after so a good start is to get 10K lights and suppliment with actinics. This will give almost all the required light that your corals need and also gives you a crisper blue light in the background to make the colours more vivid.
Your friend is right (partly) that in the hieght of day the sun is at its strongest and thus the blue spectrum of light will penetrate deepest at this time. But as i have already mentioned, this actual light does very little to hepl growth etc so it really doesnt matter tha tmuch. Running these lights alongside halides during the day has the benefit of making your corals and fishes colours more vivid. but both corals and fish also benefit from a low lighting period as this gives the fish time to finds their beds for the night etc.
Personally i would run actinics for an hour before the halides, then keep them on all day and leave them on for an hour after the halides go out.
At the moment i dot have my actinics connected but when i did this was my lighting period.
11am Actinics turn on
12pm Halides turn on
10pm Halides turn off
11pm Actinics turn off
10,30pm Moonlights go on
11.30am Moonlights turn off
