@Rob.D gave great advice on mounting your frags. In regards to only having so so success, I was in that same boat about 18 months ago. I had always been very successful with softies and LPS but never SPS. For me, it was a combination of issues. One issue was I did not have adequate par. I just assumed I was running enough. I use kessil lights also, and while they are great at providing wide, even coverage, they are not necessarily power houses. You should borrow a par meter to measure your levels in your tank if you haven't done so. Softies and LPS don't require near tge intensity that some SPS need to thrive.
I recently took PAR readings and was surprised. We have (4) Kessil 360's over a 6' tank and thought we had AMPLE lighting. What we found was far less PAR than we thought. We've been adjusting intensity up by 5% weekly. We were at 50% peak and now we're at 85% on a 13 hour cycle. We also lowered the lights 2" to 5" above the water. We've considered T5 supplements, but really don't want to get into bulb changes, etc. that we thought we were getting away from when we went from MH to LED. Hopefully, driving the intensity up on the Kessils will provide what our corals need.
My second area of change that improved my sps success is increasing internal flow within the tank. This will be harder for you to do if you are keeping a mixed tank, as it can be difficult balancing lps and sps flow needs. I run around 40 to 50 times tank volume of internal flow within my tank, which is probably 2 to 3 times higher than I ran when i didn't keep sps corals.
We have 1200gph turn through the drip trays / filter media and we have been running (2) Jebao PP8 wave makers for 15 months. We added a gyre type pump (Jebao CP-150) about 3 weeks ago. If the specs are to be believed, that would equate to 93.6 times internal volume and 9.6 times 'sump' flow. ALL corals responded very positively within a week.
Lastly, maintaining stable water chemistry is very important for sps. Maintaining a stable alkalinity level is key. I think appropriate nutrient levels such as nitrate and phosphate levels are important but less so than a tank with large alkalinity swings. I've had my nitrates as high as 20 ppm and my phosphates as high as 0.30 ppm and my acros are just fine. But I try to keep my alkalinity level absolutely as stable as I can via testing and adjusting my dosing levels.
We started dosing BRS 2 part January 2018. Initially, we were targeting 10dKH, but found our corals didn't like those levels with low nutrients and were advised to target 8-8.5. We've been there since April. One change we're considering is either going from Reef Crystals to Instant Ocean or, at least, mixing IO & RC 50/50. RC mixes to 11dKH. So, our water changes equate to an Alk spike and we have to turn off the doser for 2 days. If we use IO or the combo, we're hoping to eliminate the spike.
Our nitrates stayed at ~5ppm until Sept. We bought a new test kit because our last one was depleted. Same brand (NYOS), but the results went to 20-25. We verified the result with API, Red Sea, & Salifert. They were 20... oops. We started dosing Iron (because that tends to ramp up macro algae growth in our refugium) and began vinegar dosing. Our nitrates remain a little high, but today they are 12ppm.
Phosphates are controlled partially by our refugium and also by GFO. We target and maintain .03-.06.
We do have some cyano bacteria that shows up in a few areas. I usually brush it off with a toothbrush 2-3 times per week. It's not widespread and seems to be resolving as we gradually increase vinegar dose.
As far as spacing, I wish I had given more room for my frags to grow out. But the ones I placed close together seem to be fine. I just appreciate the colonies that look more full grown. I'd say leave at least 3" spacing but 5 to 6" might be better. If you have a playing montipora, you may want to leave even more spacing. If you are placing stags, place them towards the back. Otherwise when they grow tall, they will block your viewing if placed towards front of your tank.
