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I have a moderately planted 40 gallon tank. I have a co2 injection system
and the ph is around 6.8
ammonia: 0 nitrite: 0 and nitrate around 12.5 according to my test kit. I
have discovered that my tank actually reduces the nitrates in my tap water
which are at around 25ppm, but that's another story...

Everything is going good, plants are producing an incredible amount of o2,
described as the champagne effect as I read somewhere.

Anyway problem is I have a strange white oily film on the water surface, the
origin of which is baffling me. It just appeared a few days ago out of the
blue, the only difference recently is that the plants are producing a lot
more o2 than they used to.

Any ideas are most wecome...

JFM