Answer:
Decay (something rotting) is the cause of the film on the surface of the water in your tank. You are obviously not caring for the water (or fish) properly, and this will result in you having sick/dead fish. Please follow these simple, basic rules for successfully keeping fish. The rules are. :- 1 inch of fish needs at least 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water replaced every week. Follow the rules and the fish should be OK. Fail to follow the rules, and I can guarantee that your fish will be constantly getting sick and will live a shortened lifespan.