Hmmm... I'm still not sure what the cause of the oil is. All I do is change 25-50% of the water in my turtle's tank(s) twice a week (using tap water treated with aqua safe), and replace the filter cartridge once a month. I also scrape algae and mineral deposits off of the sides and bottom with a razor blade when it builds up a bit and do occasional water changes in between if the water is more messy than normal. I've kept many different types of turtles for a long time this way and haven't had a problem yet. I would occasionally get an oil that sounds similar to what you're describing in an old snapping turtle tank but that was just because I didn't keep up on the partial water changes and would leave food in the water longer than I should have. Good luck and keep us posted.

-Joe-
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0.1.0 Colombian Red-tailed Boa (100% Het. for albino)
0.0.3 Western Hognose Snakes
0.1.0 Plains Garter Snakes
1.0.0 Red-sided Garter Snake
1.2.0 Barred Tiger Salamanders
0.0.1 Red-eared Slider
1.3.0 Mice
50 million Guppies
50 million Crickets
50 million Mealworms/Beetles