What I have found to be the key is making maximum use of natural biofiltration. BTW, this will be part of the discussion at the August meeting. 

The WMAS had a visiting guest speaker many years ago that taught us how to use Macroalgae in an RDP Refugium. He said that in building the first ever and now longest running, marine ornamental fish hatchery, C-Quest, in Puerto Rico, (now in Wyoming), he came to the conclusion that growing algae in the system, along with the use of AC, was a great substitute for a skimmer. This is how I came to have my first Macroalgae - AC filtered system. Using AC is fairly simple. Growing algae is more complicated. Algae grows best under bright light of the right spectrum. At this writing, VHO or HO fluorescent lighting with spectrum emissions from 5000-12,000 K is the best substitute for sunlight.

BTW - by the way

WMAS - Wasatch Marine Aquarium Society

AC - Activated Carbon

RDP - Reverse Daylight Photosynthesis

VHO - Very High Output

HO - High Output

Secondly, those Red Sea Berlin skimmers are sorely ineffective, inefficient and problematic. One of my maintenance accounts has one and, like yours, it was hardly working so I unplugged it 4 months ago. The tank is doing wonderfully and looking as good as ever.

Okay, now that's out of the way, let's look at the basics. The key to most skimmers is the Venturi, as badfinger said. The Venturi is where water rushes past a plastic part with a small opening sucking in air as it goes. The tiny opening where water rushes by and air goes in must be cleaned periodically with a hard metal wire, small screw or small drill bit. Be careful not to enlarge the holes but only dislodge the calcium buildup. The plastic part has a nipple for attaching the air line. Without an airline leading up to open air, it could not suck air in, as the Venturi is usually underwater. The Red Sea Berlin is a very touchy skimmer. Not only does the venturi have to be perfect, but the pump has to be just right and the water discharge outlets have to be tuned in perfectly as well. If I were you, I'd trash it and if you absolutely must have a skimmer, save your money and get one for a good price here on the forum.

In the meantime, feel free to post in a new discussion thread, or send me pics of your tank and sump/refugium. Email pics to mark@marksreef.com or send pics to 435-632-3366. I can help you learn how to maintain your tank beautifully, skimmerless. 

Mark