I started this 10 gallon nano reef, and wanted to run a skimmer that I bought years ago but never used. It works well, in fact, too well. It foams to the point of boiling over through the cap and on to the floor.
(Solution: Dial back the air flow.)
Here, tank water is flowing to the empty skimmer from a small powerhead (green hose), but the air is disconnected. The larger, clear return hose empties near the tank heater.
Side view just after starting the air. Notice that too much foam is percolating, and the cup is already starting to fill.
Note that I will be using an old 18 gallon "tall" aquarium as a sump underneath, so the skimmer will move from here and hang from the side of it instead.
This is actually about the height the foam should be, however you can see where clear water spilled into the collection cup from earlier.
When the cup fills, the liquid will run back down the center tube. However the bubbling action can get so severe, that foam pushes through a vent hole in the cap (not shown in this photo) and starts going everywhere.
(Update: With use, the foam head is dirtier and stiffer, oozing gradually into the collection cup)
In this close-up of the connections, you can see a 2-to-1 gang for the air hoses in the upper right corner of the photo. This unit has valves for the air flow, which I now use to manage the problem.
Here, the cup is completely full, and thus the skimmer is accomplishing nothing. Just like in Ghostbusters 2, as you get mad from the constant hassle, the slime gets bigger and meaner.
(Update: The skimmate is always much darker and nastier than this)