Doubt if the bubbles are too small, the theory is the smaller the bubbles the better because of the larger surface area, so if anything you have done the skimmer a favour. If the bubbles were too small they would pop in the riser tube. Give it a day or two, there may be very low nutrient in the system.I removed the powerhead it came with and added a slightly stronger pump. Then, I removed the venturi and modded the pump to rather suck air in to be chopped up by the impellor. The resulting bubbles are much much finer than with the opriginal configuration. I was thinking, can the bubbles be too small?
There are a few new skimmers coming in for the Nano tanks or you could try the AM bio star, ran a 4ft off one and was really impressed at the performance of the little skimmer. Would also supply enough space to add carbon directly after the skimmer if you remove the bio sponge.