Could it be coming off of some ornament or piece of equipment in your tank? How long has the tank been set up and what type substrate? I had some ceramic driftwood that shed all its paint over a year or so - never effected the fish, but it was a nuisance.
What kind of filter are you using ? I have a canister and hob in different tanks, both produce micro bubbles under the right conditions, that look like debris floating in the tank.
