I would never question the efficiency of a skimmer. But to place it on the JBJ 12 gallon is not pratical. First, since the tank is only 9.5 gallons to begin with, water changes are not hard and should be done religously each week. This will help with cleansing your system and keeping in good trace elements.
Secondly, The chamber in the back has room for the placement of Chemi-pure that by nature removes impurities from the tank, it also has resins that help promote growth of beneficial bacteria.
Thirdly, I believe that a skimmer in that size of a tank is overkill, I'm talking about the efficiency of removing too much from the tank. Some of the floating nutrients in this small of a tank are needed by the filter feeders.
When I do a thorough cleannng once a month I only run my Micron filter just 20 minutes any longer and thats alreadly pulling out too much nutrients as well as it picks up the debris. It's just not practical on such a small tank that could be properly maintained by water changes.
If you were to have a fish only tank than it might be okay, but there is no where to place it in, only that you smash the canopy in the back and this would render your fans useless. And you know about the overheating issue in these tanks?
So I being an owner of the 12 gallon cube, would put a big VETO on a skimmer on a JBJ 12 gallon tank.
Mike