You have 8 molly fish in a 30 gal tank and they are developing a film on them Why?

The rules of successfull fishkeeping are :- 1 inch of fish needs at least 1 gallon of water. (A Molly is at least a 3" fish) :- Every tank needs a fully cycled filter running permanently. :- Every tank needs at least 50% of its water replaced every week. Mollys need to be kept at around 75F in neutral to slightly alkaline water pH 7. to 7.2 . One teaspoonfull of aquarium salt per gallon of water will also be good for Mollys. Mollys should be fed as much dried food as they will eat in 15 minutes each morning and evening. If you follow the above basics your Mollys will live a reasonably happy life. If you miss out on any of the above I can guarantee that the fish will be constantly getting sick and will live a shortened life span.