I was wondering if anyone could tell me what this oil looking thing is on the ponds surface? I've gotton it before but at the time didn't know how to post a photo. I areate 24-7 and it's only on the surface near the edges of pond. Seems to come and go throughout the year.Water always seeping into the pond and always going out of the spillway.It seems to always drift towards the spillway and then it'll disapear untill next time.. Everthing in the pond seems to be healthy fish,frogs, and plants..