White Foam

[madmom]

My middle son took the 10 gallon I was going to use as a quarantine tank and set it up in his room. He thoroughly cleaned it out using warm water and paper towels (no soap).He added red gravel and put in the extra filter I had running in the 20 gallon on Friday. Today on the top of the tank is a thin layer of foam. I checked stats and they are as follows Nitrate:20; Nitrite 0; GH75; Alkalinity40 and pH6.8.
The foam has no detectable odor, What can it be and what is causing it?