We've all been there at some point.
It is very common for people to over-feed fish because the cheeky little beggars are always interested in food. I don't have any experience with dwarf rainbow fish, but I do have a few different types of cories, and they spend a lot of time rummaging through the sand. All my other fish seem to know when it's feeding time, but will also act in a similar manner whenever I go near a tank (until they realise no food is materialising, then they go back to general fish stuff).
I think that the general rule is that you feed a fish a portion of food approximately the same size as it's eye, which when you think about it is tiny, and never seems to be enough. However, in nature, they would expend a lot more energy looking for food, escaping predators, etc. and wouldn't have a regular food supply either, which is why they spend so much time looking for food & eating.
Over feeding can cause problems, not just with water quality, but also fish health, so it is very important to get things right, and why we all get a bit concerned at the beginning.
What is the nitrate reading of your tap water? I'm aware that most tap water contains some nitrate, and my tap water readings are at the top end of the allowable range, so just checking.
You have only had the fish 3 days, and the tank is under stocked. If you did a fishless cycle following the method on this site, which allows for a fully stocked tank, then you have more than enough bacterial to cope with ammonia and nitrites, so I would expect them to be 0.
It think that the film on the surface of the water is possibly from excess food, or perhaps fish poo, or similar.
I've attached a link which may give some indication as to where it could be coming from, and what to do about it.
https://www.thespruce.com/getting-rid-of-aquarium-oil-slicks-1381221Please keep us updated on your progress.