here is what a few people from the usenet says:
1.) The oily substance at the surface is likely the result of oils present in
commerical fish foods. Typically not a problem as long as you maintain your water
changes. You probably are overfeeding....everybody does

Just watch the
nitrate level, 20 ppm is a good number for FW, but can go a little higher without
stress to the fish. Nitrates are a relative indication of overfeeding and
cleanliness of the tank.
2.)Bateria that has died forms a oily skin on the surface of the water, this
are the bacterial 'bodys'.
3.)A protein surface film is said to possibly reduce the oxygenization transfer
but is otherwise harmless.
4.)I found that some foods that are particularly oily do this in my tank. I
feed my breeding guppies specialised guppy foods and two of them have high
oil content. what the guppies dont eat leaches the oil into the water and
you get a slight film on top. As long as your not overfeeding it shouldnt
be a problem and does clear. you might want to consider another food if you
have labarinthine fish in though, like gouramis and betta's. The film can
cling to them when they come up to breath and cause problems later on if
they get a buildup on them.
5.) It's a protein / oil film most often seen in planted tanks. The common methods
of getting rid of it are surface agitation, a surface extractor, or just
floating a paper towel or newspaper on top of the water for a moment. for
such a small tank, I'd just go with a paper towel every once in a while.
So there are 5 different response. I never had this problem, so dont know what to tell you. It doesnt cause a problem though, as the other poeple have said.