I'm in the midst of a new build and although I'm not a scientist by any stretch, I've observed some of the same findings that you have in my last tank. I'm sure that I'll get blasted by someone with more knowledge than myself, but here goes anyway...
One thing I've noticed over the last 5 or 6 years is a move toward more minimalist aquascaping with much less rock than previously recommended. With this decrease in rock, and increase in the efficiency of other filtration methods such as fluidized media reactors for GFO/Carbon, aggressive carbon dosing, and better, more efficient protein skimmers we've caught up with the ability and timeliness of bacterial beds that colonize on rocks to do their job. Not that it's a bad thing, but it is my OPINION that nutrient breakdown and export is outcompeting and surpassing the biological filter's capacity to slowly break down wastes that could have otherwise benefited our corals much faster than in years past. Proof of this can be seen when someone that had a thriving refugium decides to start carbon dosing. The chaetomorpha algae most likely will die off in a couple months. To me, this is an indication that many nutrients are being stripped to the point where they simply can't support some life. More importantly, they're being stripped at a much faster rate. As the cycle continues, the sterile environment minus essential nutrients can reach a critical mass that effects our SPS corals.
I'm not saying that nutrient export isn't important, but we may have reached the point where it's happening too fast. With this sterile environment, many of our corals are starving for food while we keep pounding them with light and tweaking our photoperiods. Slowing down the breakdown of nutrients may be a better and more viable option than depending on photoperiod manipulation. Run a little less carbon through a reactor, ditch the GFO for a while or maybe even consider eliminating carbon dosing all together. As the clarity of your water increases, obviously so does light penetration, which wouldn't hurt since you're reading 500 PAR on the sand bed. This suggests that you have a tremendous amount of light in the upper half of your tank, which most Montipora species would stress under. Many of us hear the word "nutrient" and assume it to be a bad thing as it relates to our reef tanks. Truth be told, our corals need them to grow, color up and be happy. If you starved yourself of vital nutrients, you may look thin and in good shape for a while, but slowly your hair would turn wiry, dry and coarse, your finger and toe nails would discolor and you'd have a generally weak feeling, not displaying a "healthy glow". Regardless of debates pertaining to whether SPS corals need to be actively fed or not, the truth remains that polyps are arms and mouths that are searching for food, and I myself had better luck when I was feeding them something than when I was starving them to sustain low nutrient, clear water.
In a high light environment that never registers any indication of nitrates, I think other forms of feeding the corals at night would be a much more beneficial course of action, which I witnessed myself without having to play with a photoperiod while running 400 watt Radiums on Galaxy ballasts in Lumenmax Elite reflectors. While there is extremely high light on tropical reefs, it is actually a nutrient rich environment with immeasurable levels of planktonic life that feeds the corals all day long, regardless of light intensity from a clear, overcast or stormy day. We simply can't replicate these conditions in a captive environment so we have to supplement what is missing. Our corals are usually hit with the same amount of light from a fixed position every day, so the lighting portion seems a less reliable variable to play with. Others above have listed some excellent food items. Some people say just feed your fish more. I've stopped subscribing to this approach for a couple of reasons:
1. It's extremely indirect and IMO inefficient. Fat fish with big poop don't necessarily make a happy reef tank. Targeted coral feedings in moderation are probably more efficient.
2. If you begin feeding your fish more you may eventually have phosphate issues that will rear their ugly heads in the form of nuisance algae and tissue recession from the base of the coral. As the fish grow, they'll require more food to produce the same amount of waste to "feed the corals". Feeding a 5 year old child the same baby food portions from when he was an infant won't make him produce as much waste...same goes for growing fish. Feeding them considerably more daily will obviously make them grow faster, and I'm not a fan of quickly testing the filtration limits of my reef before everything goes haywire.
I heard somebody once say that reef tanks are like race cars.... the faster you go the harder you crash. There are many others on here that use 400 watt bulbs with excellent results, so we know they work well. It's the other components that aren't as interesting and visually appealing to tweak that more often than not make the difference (what/when/how often we feed, filtration, flow characteristics, maintenance schedules...etc.)
Just my $1.50
