I don't think feeding has anything to do with it. The Nitrites in your tank are building up for some reason and it has to do with the filter doing a mini-cycle for some reason.
Common causes for a mini-cycle are too big a water change at one time, done over and over soon after the original cycle; too heavy a load of fish for the tank or cycle status; use of a chemical or medication in the tank that has disturbed the status of your filters biological filtration; replacement of the gravel in the tank; replacement of the filter media in the filter with a new cartridge without leaving something to seed the new media. I do not know if any of these apply to you, but it does NOT seem to be that your tank is overstocked so that is probably not a factor. Sometimes however it takes from 6 - 8 weeks or longer to cycle the first time so it could be that you have not ever quite gotten through the cycle process in the first place.
If you are still cycling the first time, (see
http://www.fishlore.com/NitrogenCycle.htm) then you are smack dab in the middle of the process with Nitrites showing and no Nitrates yet. This process is hard on fish and some are not hardy enough to survive the process and it could be that since you have fry that they will not survive due to the stress on their systems.
When I said keep Nitrites at bay, I meant that they need to be kept at 0 but if you are still cycling it is not going to happen. The best you can do is test frequently and change the water often enough to make your little fish as comfortable as possible. One suggestion, when you change the water wait a while before you test or your readings will not be accurate as the water will not have had a chance to mix into its final composition. If you even wait a few hours it will give you a more accurate reading. Overnight is probably even better.
Rose