Hmm...may seem like a stupid question, but is the tank an open top tank? Is there a lid? It could just be particles from the air, like dust haha.
Or, if you have hard water like me, the calcium that builds on the glass can get dislodged and do that, float at the top then eventually sink and dissolve. If its not alive and it sinks after becoming water logged, I doubt its something the aquarium is producing, but I'm no expert.
I would just recommend putting a lid on your tank if you don't have one already to stop any debris from coming in.