How Much Food? Nitrate Question.

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-1381221
Please keep us updated on your progress. 

Thanks, I tried the paper towel method which worked a bit but it's still there today. I will try the glass method today! My fish seem to be settling in, the rainbows came up to the surface for their breakfast today and have been displaying their mating colours to each other too. I think I have too many males though that's a problem for another day!
My nitrate in the tap reads as 0 on the liquid test kit, but I get a slight colour change on the strip tests I have. The test kit is fine as I had lots of nitrate when cycling. I'm glad because I have a seven day holiday coming up I won't have to worry about the water quality.

My tap water has between 0 and 5 ppm nitrate, just to make you even more jealous  It comes from a reservoir high up in the wilds of Northumberland.

My tap water has between 0 and 5 ppm nitrate, just to make you even more jealous  It comes from a reservoir high up in the wilds of Northumberland.

I live in Northumberland and the water comes from the Kielder dam reservoir which is also likely to be @Sue source.   Water quality is no issue at all.  However, it's not all good news.  There isn't a great population centre past Newcastle so finding a good fish shop (that doesn't batter) is impossible and even around Newcastle it's difficult to get rarer fish further North than Durham.  Getting fish shipped can be so hit and miss.

Anyhoo, back to the topic at hand...  As I responded today to another of your posts feed the tank until you have it stocked fully which you want to try and do asap.  By fully stocked I mean the amount of fish you want to put in and not how many fish the aquarium can handle.  There'll always be differences of opinion on what stock your aquarium can hold based on filtration, how messy the fish are, if the fish are territorial and so on.  I personally use the AqAdvisor stock checker and then knock another 15-20% off as I'm conservative these days with stocking.  It's a personal choice.

Flake is the greasiest fish food I've tried so far especially the Tetra Colour one and tend to release some oils before sinking.  Using a filter that breaks the water surface with a decent flow or an air stone will stop the film supressing gas exchange.  Other than that it is mostly harmless and a bit unsightly in a tank where the surface can be seen from above.

The rule about feeding the size of their eye is a general one based on the size of the fishes stomach.  It's good for omnivores who get a mixed diet with lots of goodness.  Grazers like vegetarian plecos, cories, otos, etc. will eat more to get the same amount of energy from their food.  If you ever keep full on predators they require less regular feeding but large amounts and can pack away way more than their eye size.  All fish (that I know of) are opportunistic and will over eat.  This is because in the wild they never know when the next meal will be and it's an instinct they keep.  Generally look at a care sheet for the fish for which diet group it comes under and equate it with a land animal to male more sense.  For example, cows need to eat lots of grass, us humans just little balanced meals regularly, a lion will eat a lot in one meal until it can catch the next one.

The fish will be producing waste.  Doing a fishless cycle means that the bacteria of the completed cycle can handle way more than your tank is currently stocked at so shouldn't register on any tests.  As it's been three days I would be cautious as some of the bacteria will have died.  Gradually build up the amount of food going in based on your next additions now.  Other than that it all seems to be going as it should (except the brown diatoms for which amano shrimp will be your best bet but only if you're not feeding other algae foods).

Thanks, it probably is the flake food that has caused it then, which I'm not using now anyway as the fish don't seem to like it much! I didn't want to add all my fish at once incase something went wrong and they all died. I had read about the bacteria going dormant (rather than dying), I won't be getting anymore fish until Tuesday at the earliest - that will be a week since my cycle completed and fish went in, so I think that will be ok. I need to add some more female rainbows and find a nice pair of fish as a centrepiece and I'm done. I too like the look of a tank which isn't too crowded and I already get loads of enjoyment watching the fish I have they're so entertaining 

I have just been using Aqadvisor to look at the stock levels for my river tank before I make my final fish list.
We've previously noticed the difference between the stock levels between the two sites, so I'm hoping that the more experienced keepers will answer your question, to save both of us from searching through previous threads.

However, as far as your particular question is concerned, I really wouldn't worry about 101% on Aqadvisor. That's a tiny amount to be "over-stocked". I'd be more than comfortable with a stock level of 65% from the CC. Most of my other tanks have been stocked based on the community creator here, and they've been stocked at around 90% by the CC, and been fine (long term levels of ammonia = 0, nitrite = 0, nitrate = same as tap water).

Also see: http://www.thinkfish.co.uk/article/stocking-levels-for-tropical-aquarium-fish