1. Make sure your temperature is stable... within 1*F. Temp swings cause a lot of issues with SPS and are usually overlooked. fans, heaters, chillers, whatever it takes for temperature control - this is important with SPS success... knowing what your temp is at all times and monitoring it carefully. Temperature swings can cause major problems.
2. Make sure your salinity is stable... have an auto top off system to make sure that your salinity doesn't swing. This is the second most overlooked quick fix. Use a refractometer to know exactly where your salinity sits. Correctly calibrate it, and then add it to your weekly maintenance routine. Pick one day a week that you can dedicate one or two hours towards maintenance, including a water change, water tests and even allocate 30 minutes for careful observations. Heck, even start a small journal.
3. Alkalinity is a big killer for SPS corals. If you don't have a test kit, get one. Make sure that you're keeping your alkalinity and calcium and magnesium in check by adding the correct dosages from whatever potions you are using. IF you don't dose, that's perfectly fine. You probably don't have enough corals in the tank to even warrant dosing anyways. So, weekly 10 to 20% water changes is necessary - by also removing detritus out of the system and keeping your filters changed and clean.
4. Built up detritus. If detritus is building up in your tank, in your sandbed and everywhere else, it's time to clean it all out, blow off all of your rocks and get your tank sparking clean. After removing as much detritus as possible, take your sump out and clean all of the muck that has built up in it and redesign your filtration system where ALL of your water has to pass through a filter sock or two in order to reach the sump. Keep your filter socks changed out regularly. Removing detritus from the system will actually make your nitrates and phosphates decrease pretty easily and quickly.
5. Change your lighting scheme. I'm starting to realize that it's not the quantity of the light but the quality of the light that makes a big difference. PAR readings with LEDs basically mean nothing at this point, since it is so completely inaccurate. Some LED fixturs have low PAR readings but a 3:1 or 4:1 PUR reading. PUR is Photosynthetically Useable Radiation... Which means that your fixture might put out 4 parts PUR to 1 part PAR... this is what makes LEDs extremely efficient. Dial down your LEDs and follow the guidelines on the Reefbreeders website. Once you have all the corals that you are ever going to have in the tank, then you can start increasing the intensity by 10% every week. Most people end up only running ReefBreeder LEDs at 70% of total intensity at MAX.
Once you have taken care of these aspects, then you should start to try adding frags again to the tank. Get yourself some accurate test kits - especially calcium, alkalinity and magnesium test kits. Knowing where your alkalinity sits is one of the most important results that you have got to keep up with, especially when dealing with SPS corals. If you have any corals already in your tank, make these changes gradually. Making any large adjustments in the tank very quickly can cause severe problems. do some detritus control one day, ... a water change the next week, change out one filter and leave in the other for a few more days... just change your routine a little at a time over the course of the next month or two.
One indicator of a clean and well maintained tank is the presence of coralline algae. If you can get your coralline algae to grow well and thrive, then you can get your SPS corals in your tank to do well too.
Finally, we know that reef nutrition plays a very important part in reef biology. Our oceans are rich in food but poor in nutrients. Meaning that corals have lots and lots of live foods to eat, and do not have to deal with levels of nitrate and phosphate that we do in our own home aquariums. Providing the proper source of nutrition to each coral while maintaining low levels of nitrate and phosphate is key to dealing with SPS corals.
I have learned that a KISS method is probably best when dealing with SPS Corals as well... keep it simple, stupid. lol. Make sure you are doing all the weekly maintenance routines, check your salinity, temps, alkalinity, calcium and magnesium. Observe your corals - watch for browning over time or bleaching over time. (Browning is from too much nutrients, bleaching is from not enough nutrients.) Rapid bleaching or RTN - Rapid Tissue Necrosis is still not fully understood by aquarists or scientists and can only be speculated upon. I prefer to think that it was just meant to be. lol. I don't have the tools necessary to study it at a level needed in order to understand it.
That powerhead of yours acts like a squirt gun. I wouldn't aim it at any coral at all... only passive water movement is necessary with montipora - only enough flow to guaranty that detritus does not settle on top of the plate.
Generally when dealing with flow and SPS corals,... a lot more water movement is necessary for thicker branched acroporas and then a lot less flow is required for the smaller, more delicate thin branched SPS corals. They live basically in two different environments and have different flow requirements. We as aquarists like to keep a bunch of different species of corals in the same box and sometimes it just doesn't work that great.