Strange Liquid In Tank Less Dense Than Solution

I've an NFT setup consisting of 3 tanks. 2 of them are fine. However 1 of these has a whitish colour liquid floating on the solution in the tank. This is similar to the way oil floats on water (because it is less dense). I've no idea what it is from. The plants look healthy and the other 2 tanks are fine.

does the water (at any point in the water fall) have access to HID lighting? I sometimes get shit floating in my hydro reservoir, but I bought a little fish net on a stick at Petco and skim out all the crap if it needs it, and make sure to change out the water once a week or 2...and I'm fine! What temp is your nute/water solution?