Larger tanks with overflow system don't have such problems cause the surface water gets filtered. The bacteria built up in the filter sponge can work wonders to the oil particles. In less than half a day, your tank will be without the oily surface. This is a good example of the usefulness of the hospital Tank . It is recommended that you use salt, before you try anything else for parasites.
Salt is recommended because it has so many benefits over other medicants.
It does not get bound out of the system by organics or sunlight. It does not pose a health risk to humans contacting it. Remove submerged plants. Perform a fifty percent waterchange, and clean the tank as well as reasonably possible without causing undue delay in treatment. Add all at once in the case of epidemic mortality.