How Much Food? Nitrate Question.

How do I know if I am feeding enough? I now have 7 dwarf rainbow fish and 8 Corys.
It's only day 3 of having actual fish, but my nitrate reading (and ammonia and nitrite) is 0, pH is normal. How do I know they are getting enough? The rainbows are always hungry, I've fed them all twice a day the corys get around 10 sinking pellets, and 4 bottom feeder wafers (not algae) per day and they pick up the flake which the rainbows turn their nose up at. The rainbows don't like the flake I have, so I got them some micro pellets which they love but they are always hungry it seems. My tank may be understocked it's 125l but surely there should be some waste going on, I have seen a few of them poo.
I'm going to do a small water change today as there is some uneaten pea (a hit with the rainbows but once it's on the floor they lose interest) on the substrate and there is a weird film on the surface of the water. Just waiting for the corys to finish their breakfast.
Thanks, sorry if I seem neurotic! I told my husband this is like having another child!