Green film on top of water

I have a heavily planted 66 gallon tank that has been setup for a number of
years. Lately I'm experiencing a greasy green film that floats on top of the
water. All the tank parameters' are ok. No problem with nitrates, or
nitrogen. I do a water change roughly once every 2 weeks and change about
30%. I try and "soak up" the film with paper towels and once the water has
been removed, I wipe the inside of the tank to possibly eliminate any
additional film to stay around. I notice that the film reappears in about 4
days. I've tried filtering the water, fertilizing the water, etc.

Suggestions? This one has me stumped.

Thanks.
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_____________________________
Later.
Doug
Have a better one.
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