Turn off the filter and watch them. If they float up they are air bubbles, if they settle then they aren't. I had trouble with really fine particulate matter in my tank until i inserted a filter bag (tip of a nylon stocking also works) filled with polyester filter medium in the filter. It does a great job of filtering out really fine particles. If that doesn't work it is probably bacterial, and will go away once the infusoria population (tiny animals that eat bacteria) catches up to the bacteria population.
If you already have a filter with this type of media, then just give it a few days.