oily betta water

Question
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Followup To
Question -
I have a large male betta, have had him for about a year now. I was keeping him in a half gallon bowl, and recently transferred him to a split 10 gallon aquarium. When I had him in the bowl I would change half his water every week, but still wound up getting an oily film on the water. Now that he is in the aquarium, the same thing is happening, but only on the side where he is kept. I rinsed everything that went into the aquarium thouroughly (except for him, of course!)The tank is divided by a screen and I keep a female along with a couple of guppies in the other side, and this film does not cover that side. I generally skim it off with a clean cup, but it will be right back within a day or two. I was wondering, could this be caused by something that he produces naturally? It does not seem to bother him, he always has been very active and energetic, a strong hunter and good bubble blower, but it looks nasty. I have one undergravel filter in the tank.
Answer -
Hi Sara,
 I assume that when you say "oily film" you don't mean his bubble nest, right?

 Do you feed him the same stuff as the fish in the other half and does he eat the food or is some of it rotting?

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>

Right, it is not his bubble nest, although it is in his nest too, it covers the whole top of his side of the tank, and really looks similar to an oil slick. I do feed him and the female betta pellets, and give the guppies a small amount of flake food (they seem to have trouble with the pellets), and I also keep a couple of ghost shrimp in the side with the female to clean up any leftover flakes. If there are any pellets sitting on the bottom that the bettas don't eat I remove them before they can rot.  

Answer
Hi Sara,
  Hmmm... it must either be coming from the pellets or something in the air.  Let's hope it isn't the latter because you are breathing the same air.  

  Actually, one other possibility... do you have an air pump hooked up to the tank?  Their could be some oil coming out of the air pump, into the air line and then into the tank...

  My best guess would be something in the food, but then that doesn't explain why you don't have the oil on the female's side...   

-- Ron
  rcoleman@cichlidresearch.com
  Cichlid Research Home Page <http://cichlidresearch.com>