the film that you talking about is fish oil and not going to hurt your betta. I have it too in some of my tanks. I try to take it aut with a spoon. Well it not really helps though ..it breakes on little white pieces. I do a lot of water changes so i don't care.
Once the nitrogen cycle is established make sure you do regular 50% water changes with gravel vacuuming to maintain water quality. Your tank is small i suggest to change the water 2 times a week. Well it just me i think clean water keep bettas healthy.
Sorry if i am repeating myself. I don't have filters but i know you have to swish/rinse them in the bucket of old tank water with a water change a couple of times a month- best to wait until the cycle is establish before you start this unless the water flow is compromised.
If you can, have your water tested at the pet shop weekly and once you have Nitrate 5-10ppm with ammonia,nitrite 0ppm-you are most likely cycled-this can take 4-6 weeks...
I am personally (sorry Hallyx) think 90* is to hot. Not all betta love 90* 78-80* is good temperature.
Not sure if this link can help http://www.bettafish.com/showthread.php?t=99717