10"--I can relate with much of your situation, I have the same skimmer, different tank (see sig) and a 10gal sump, with everything comes to about 25 gallons. I do not think the skimmer is really the issue. I had a 10 gal with 175w mh in a pendant style reflector, kept it about 12" off the water, everything was great with growth/color etc. all stony corals/sps. I moved it all to the current tank, same rock, more flow but upgraded to a luminarc mini. Got massive bleaching initially, I had to move the light up to around 18-20", and it stopped and what I have kept is starting to turn back to the way it was. But visually it looks much dimmer than my old tank?
I am thinking about possibly changing my lights to a higher K, sinse i do not use supplimentation and I keep my lights on for 9hrs. I currently use a 175w Hamilton 14k.
The skimmer works but I find that if I add water directly into the tank with the skimmer, it will kill it's production for some time. If I add it to the tank connected to it, it will not kill it's production (make it not foam).
Also, with my smaller tank I had imo, poor color/growth/minor stn for some time but could not figure out why. I found that from day to night my temp was swinging a few points. Sinse i couldn't keep it even at lower temps I decided to raise my heaters to hold it at 80 (prior it would hit 80 in the day then drop to my heaters set-78 at night) This made a very fast drastic difference in sps color/growth/and stopped stn issues in that tank. Also, I use to allow some minor salinity fluctuation from time-to-time (in small tanks 1 gallon can throw you a couple points if not watched, so can evaperation). After finally giving in and automating the water top-off and getting top-off salintity ratio's dialed in this also made very noticable improvements in the tank.
Fluctuation, even very minor, is constant in small tanks like these. And most people do not relise that the small tanks can change 1pt in salinity in a single day just do too evaperation. If you find heat and salinity fluctuates, even slightly, Iwould address it. This might mean running the tank a little hotter than you would prefer or salinity a little lower.
just some info on how I made some issues for myself, everyone else kept telling me it was probably unmeasurable phos or nitrates or lack of food (I dont keep fish or feed). You might be doing it all right, but fluctuaion in good ranges is still fluctuation, and these tanks wouldnt be nano's without it
---john