One reason, I think, it might be from wood, leaking tannins or resin or something.
I have just started another aquarium recently. I washed the substrate, I put inf water, and left it for about a week. No problem, clear and fluid water.
I put in a piece of wood that had been soaking for about two months.
At this point, there is no water pump or filter installed. Just water, sand, wood.
The next day, there was a thin film on the top of the aquarium with trapped air bubbles in it on the surface. About 3 days later, the film and bubbles are still there.