Oil slick on water surface

I have had a 75 gal.hex  take for about a year. All community fish.  I recently noticed what seems to be an oil slick on the surface, otherwise the tank is crystal clear. I have had a fish or two die in the past month but removed them promptly. I tried scooping out the oily stuff and do a 20% water change with no luck. Any clues as to what this could be? ???

 

it might be a bacterial bloom.. if you wipe the surface with a paper towel or skI'm just under the surface with a cup, does it seem to break it up? How much aeration does that tank have?

 

It does seem to break up when swipped or the fish go to the top to eat. I have a fluval 204 filter system. No actual arration though. The water comes out of the filter line about two inches below the surfacewith maximum force, according to the directions of the filter it can be placed below the water surface or so it flows over. Should I move it? I have had this set up for over a year with no problems.

 

Is it clear? What type of water conditioner do you use? Look at the bottle, does it say that it treats the slime coat of the fish? It's possible that it is that part of the conditioner that treats the slime coat. If it is, then it's not a big deal, but if you want to get rid of it then try using less of your water treatment and do small water changes but more frequently. It will slowly go away over time. If it's not clear though then it's not this chemical, and if that's the case then what color is it?

 

When I did a 50% water change a month ago I used start right, according to the directions. The color was a milky white. I went ahead last night and set up a bubble strip. Thinking maybe more aeration was needed and this morning the fish seem to be happier. And obviously the oil slick is gone, but still would like to know what it was.

 

We had the same thing happen with our betta tanks, couldn't get rid of it until we added bubble stones. I read it was a bacterial bloom and something that happens sometimes with an aquarium, but agitation to the water's surface breaks it up.

 

It could possibly just be some of the oil in the food. I had the same problem until I added more agitation to the water's surface. An airstone or adjusting the water level can help with this problem. I simply adjusted the water level. If you have a HOB (hang on back) filter, when the water level is high, there's not much water movement on the surface when the water is added back to the tank. Dropping the level down a bit causes more surface movement and breaks up the oil slick where it can be continuously agitated or sucked in by the filter. The airstone does essentially the same thing by causing surface agitation.