Whitish, oily substance appearing only at the water surface. What is it?

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Aquarium Advice Regular
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Jul 30, 2010
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I couldn't get any pictures for it, due to the angle of viewing.

It looks like a small scale oil spill in the ocean, except the oil itself is white.

It appears whenever I do a partial water change in my tank, filled with one type of plant. I don't know what plant it is. It only grows leaves and roots. Leaves are like leaves from a sprouting yellow bean in a garden.

These foamy, oily substances appears near the leaves and stem of the plant. I really wanted to guess it's a natural occurrence. The fishes, when eating, they swallow the oily substances. If they are not eating, they try to swallow it, then spit it out to form a whitish (spit?), I don't know.

EDIT:
Here's a picture of the plant, in MS Paint. I don't have digital camera with me at the moment.
img_1483634_0_21236b6699d5a02c9cfbe2738ce041bb.jpg


It's not the cloudy water; the water itself is very clean; you can see the bottom of the tank from the top without much effort.

Is it normal? What can I do?
 
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