Water Chemistry

Our tank seems to have developed a slight film on the waters surface.All thr waters psrameters are good but the film doesn't
Seem to go into the overflows. Any ideas?
Any help is apprrciated.

I just figured out that my nitrate test kit was not reading right and got a new test kit and it says my nitrate levels are 20-40 ppm. I am going to do a water change right away but I qam wondering if there are any other metods for reducing nitrate. Also is this level of nitrate too high??? my gha is almost gone but there appears to be more brown algae. All the other readings seem good . Thanks Kitch40

Further to my queery about using the de humidifier water for makeup water, which I wont do (thanks to all), is 2 1/2 gallons of kalkwasser per day to use fur make up water? Because the pH is so high I thought it might be too much.