All skimmers have specific depth for the best skim. Mines said that it works better at 6inches from water.
About the algae on your rocks, is to be expected. Is a process that will be frustrating and annoying. I'll recommend to buy reactors, that way you will accelerate the process. I have 3 reactors. One for phosphates, one for carbon, and one for nitrates. You will spend some $$$ but this will be very helpful in making sure the water you are using is reading 0.0 tds. That way you will prevent increasing phosphate to your tank.
Check for the "main" parameters if you are starting your tank: salinity, nitrite, nitrates, and phosphates (I think there is one more, but can't remember now). Ca, alkalinity, magnesium, ph... you can worry about later, but you will have it when you do water changes.
Dont worry about your skimmer. If its accumulating junk, then it is working. And the sock is trapping the "dirt".
With your Radions, start with a low %. I started mines at 25% and continued increasing till 45%. That was to strong for my corals and dropped to 35%. It's 16" from the water. They are doing fine, but be careful, bc its strong and can kill your corals. I love them and so far it is doing its job. It has an option that can acclimate your corals before you get to your regular percentage.
If you have a long tank I will recommend to put the power heads at each end of the tank. Just avoid the air suction. Mine is a square 24x24x24, so I have both at the back of the tank. Also, the rocks will help you with the sand dune.
I want to make a point, all I said is my own opinion and what I have experienced with my tank. Hopefully this will help you and I would like to see (or if you have it already) a thread of your tank. Pls, keep me posted.
