Mm, it prevents gas exchange, which is pretty bad, especially during times when there is little oxygen production (i.e. at night). If you have a ton of plants and a ton of fish, then that oily film might cause the death of the fish due to hypoxia...but that's rare.
The oily film can also get very, very nasty, as it grows bacteria on it as well...
Overall though, you probably don't have a problem and can get away with removing it when you do water changes (just as a preventative) rather than every day.
The oily film can also get very, very nasty, as it grows bacteria on it as well...
Overall though, you probably don't have a problem and can get away with removing it when you do water changes (just as a preventative) rather than every day.