Anyways, most likely, if its just a bit of film, its probably just slime coat. A little slime coat shedding isn't too much of a problem. Just do as zoodiver says and wait your cycle out.
In your situation i would definitly have added some salt.
Take a look at this;
--------------------------------------------
"Nitrite binds to red blood cells and blocks their ability to transport oxygen. If nitrite is present in high enough concentration, fish blood will turn a chocolate-brown color. Because of this, nitrite toxicity is also known as brown blood disease. This effect is most easily seen in the gills. However, toxicity can and does occur commonly at lower concentrations, before gills have turned an obvious brown".
...
"Because a fish's red blood cells with nitrite present cannot transport oxygen, affected fish frequently appear to be oxygen-deprived, even in water with high concentrations of oxygen. Affected fish may gasp at the surface or stay near airlines or water inflow".
...
"The best way to avoid ammonia and nitrite toxicity is by making sure the biological filter is well established and maintained. This means having patience when first starting up an aquarium and testing water quality, especially ammonia and nitrite levels regularly".
...
"Water changes may be the only temporary solution for nitrite spikes as well. However, another temporary solution is the addition of salt (sodium chloride) in proper proportion to the tank. A chloride to nitrite ratio of 6:1 has been recommended to decrease toxic effects. The addition of chloride during a nitrite spike helps prevent entry of nitrite into the fish's blood stream".