I missed something, what are we bumping?, the actual surface skim?
Surface skimate is usually harmless, mainly amino acids from your contributions. It does effect gas exchanges. Taking into account temp coefficients, It generally takes approx 72+/- dynes to break surface tension of standard water, add to that more complex molocule strands & it that number can potentally rise.
If its something from your wood, as long is known as safe wood, your usually fine.
Even if it is something leaching, oil has a lower S/G that water, so it will rise to the surface. If you have a sump with floss or filter sock, consider it gone, same to any surface attachment that fits to a canister or something to that effect.
If your really worried about it, try what others have mentioned, or use something simple like laying paper towel on the surface for an instant lift.
Is this what your asking?