I have a couple beta fish, one in each bowl and after I clean their bowls, I rinse the rocks and the bowl with just plain water and than put the fish in but ontop of the clean water a few days later a thin layer of this clear 'film' starts covering the top layer of water... what is that clear film and how do I prevent it from happening?

It is caused by the water sitting there with no movement. If you don't want this to happen, get them little aquariums (they have small ones for the size of a beta). The filter system will keep the water continuously moving, and also you won't have to change the water as frequently :)

Fish create pollutants.
The thin layer is fish pollutants (old food, urine, excrete) mixed with dust.
The only way to prevent this is to change the water daily or, as mentioned above, get a little tank with a filter system.
Good luck and happy fish keeping:D

I raised many bettas myself. However there is a possibility that its water pollution because betta water can get easily polluted. Also the water needs to be aired out because the tap water is very poisonous to the fish (about 2, 3 days the water is ready) I hope this information was helpful.