I have a new pond that has been filling up all winter and is now about half full at about 7 ft. Early in the filling process I noticed some oil at the corner of the pond where there is a good amount of runoff. The builder of our pond said that it was probably just a little oil from the oil filter that he changed a few hundred yards away that washed down to the pond. He said that it should dissipate over time and it should be fine and this was about 3 months ago. It wasn't much but if I scooped all of it up, it would've probably been half a gallon of oil. The pond is a little over one acre, stocked 1500 CNBG yesterday. Today I went out and moved a Christmas tree that was on the bottom of the pond and I saw oil rising to the surface (rainbow film). I noticed one 4 inch blue gill dead on the bank today. I don't think that the dead fish was from the oil. I noticed a bunch of fish swimming around today in the pond that looked fine. Is that amount of oil harmful to the pond? If it is harmful, how do I fix it?