Since corals come in 2 variations, photosynthetic and non-photosynthetic, one can postulate that those are the main things gearing those corals. Since they're vital to the corals, we see 2 directions that dictate metabolic function in said corals, light or food. Though it seems each can survive a period of time with ONLY food, the photosynthetics NEED light. But as it's been discovered, they can receive light, and then internally 'shut down' their photoreceptors, and even spend energy trying to compensate. Thus this invites the premise that you need light, but not too much. If you do not approach photo-saturation, the corals metabolism continues to operate within the range defined by how much light they receive.
Thus there's a middle ground for par vs duration. Unless you can account for the geographic location of each coral, you're guessing.
So to sum it up, it's best to have lower par to avoid this maximum, but as long as a duration as you can to avoid all the other negatives. (algae growth, unwanted other photosynthetics, heat in some cases)
I myself am up to around 14.5 hours of light with a few hours peak, all the while seeing a minimal amount of algae. You WILL see algae no matter what, this is the environment algae grows in, and to try to 'beat' it by raising the spectrum to be void of all but blue, can be a very fine line between success and failure. I myself tried this, and all sps suffered or did not grow. You can definitely have necrosis events by giving the corals too little light.
It took me a few months to raise my lights to where they're at, the final point in which I stopped was when I started to see some corals pale-out on top. I theorize this is the same as approaching a 'bleaching point' in which too much light is causing negatives. After ceasing raising the PAR of the lights, I can see it's probably correct because the ones that have paled on top under the hot zones, are now growing AWAY from the hot zone.
It will be difficult to find your middle zone, but give it time, start low, and allow a week or two inbetween raising the PAR little by little.
The other points on flow work into the equation because as you raise the metabolism, the corals will have excess waste to remove, this is enabled by flow. But again, you don't want too much flow as this can be detrimental to the coral as well.
It's a multi-pronged situation, but that's why the number one attitude towards the hobby is 'slowly make changes' as it can take days and/or weeks for the coral to adjust.
I hope this helps!
I'd like to hear the results.
