How often shall I replaced the cartridge in the filter?

The only time a cartridge needs to be replaced is if the fabric rips or the end caps don't seal well. Otherwise they can last a very long time. I just replaced my filter after 10 years because the end caps no long sealed and debris was getting by the filter.

But a film on top of the water doesn't necessarily mean you have a bad filter. Do you know what the film is (e.g. oil, pollen, etc)? If oil, use scum balls in the skimmer. You can also try a skimmer sock.
 
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