Found out about this this conversation from the switch back to MH thread. I am a biochemist by education but I don't chase numbers very much. In my experience (first reef tank set up in 1988 or so) constant tweaking of many parameters at once has never worked for me. You guys have been working this for a while so if my observations are patently obvious I apologize - just trying to cast a new eye on a serious problem that I too have lived through. I read through the thread but I did it fairly quickly so if I am suggesting things that have been asked and answered I apologize. In my case I never did figure it out so I rebuilt my tank and eliminated many of the things I was doing (biopellets and GAC 24/7 as an example) going back to a more simple setup and other than the redbug issue things have been going well for over a year now. I've only recently started re-introducing GFO.
Three things I consider the most important in an SPS dominated tank are stability, light and flow. I see a lot of GAC in GAC out, polyfilter this, GFO that in this thread in a relatively short period of time, dosing nitrates, dosing zeo. Nothing good happens fast in this hobby and there is no silver bullet.
I also see kalk as alkalinity/calcium supplementation in ATO. Is your ATO capable of dosing frequently like a Tunze Osmolator. If so, I would expect your alkalinity/calcium to be quite stable but if it adds things in bigger batches that is probably less stable. I don't like kalk as an sole additive due to the seasonal variations in evaporation but I do like it in general as I find tanks respond well to it. 2 or 3 part dosing is nice easy way to get stability, especially on a small tank.
I see you have a Vortech but I don't know Vortech well enough to know what turnover you have in the tank. 40x turnover is not that high for SPS. You could probably easily go 50x+
Pests. Have you tried to take some macro shots of your SPS to see if you have pests. My acquisition of a macro lens led to my discovery that I had red bugs. Not enough to destroy but enough to limit growth and colour.
Light - I think that has been discussed fully and I think raising the lights can do nothing but help. They looked way low to me (still could go up a bit by the last picture).
Lack of coralline is interesting. I used to run 2x400W XM10K over my 120 long ago. Could never get coralline to grow above half way up the side of the tank. I assumed that was photo-inhibition but in retrospect I could have been magnesium limited back in those days as I was using IO and never tested.
Using a decent salt really should take care of the trace elements. You could also look to switch from kalk to daily dosing of Bio Calcium if you are worried about it. Again, in a small tank that probably wouldn't be too expensive.
Starvation - another potential issue - given your need to dose nitrate have you considered just getting another couple fish? Many of the TOTM have way more fish than I would ever consider and they seem to do fine. Fish poop feeds SPS seems to be the general thinking.
Adding a grounding probe to just eliminate the possibility of stray voltage will cost you about $20 - really no reason not to just do it on spec.
Toxins - there was an article in Coral magazine a while back that showed a clip on a bag (that would hold carbon or something). Looked like plastic but it had a metal spring on the inside that corroded and screwed up somebody's tank. Could be something as simple as that.
Hope this fresh pair of eyes is of some use...
For reference, this is my tank. It has been a up in it's current form since December 2012. Picture is pre redbug treatment so it is looking up now. Improved growth, way better PE.
