That film can usually be attributed to a build-up of proteins from excess nutrients and waste. This is not uncommon, but can be hard to treat. One suggestion is to do exactly what you did, add an airstone or pump to disturb the water surface. It can break up the film and the tank looks better, but the water still isn't balanced correctly. Another thing to consider with a turtle tank, is that many species don't like high water flow.
I would closely monitor the feeding and make sure the turtles aren't eating too much or leaving excess food to settle in the tank. You may also consider adding cories, or another type of small catfish to naturally remove the left-over food and waste.
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