painted turtle problem

Question
I have a painted turtle, and twice in the last week, I have found sort of a fibrous white film on the surface of the water.  It looks almost like baby spit-up.  My turtle is otherwise active and looks normal, I can't see anything that would indicate where the stuff is coming from.  The first time it happened I did a 50% water change and added a bit of Melafix.  Do you have any ideas?  Please let me know if you need any more information, I also took pictures of the stuff.  

Answer
It could be many things- but it is usually some form of proteins in the water, often the result of foods offered or the digestive process. Probably not a real problem as long as it is getting cleaned out in a timely fashion.

I'd look over the basic cares and diet at a good site like http:/www.redearslider.com or http://www.austinsturtlepage.com and just make sure I was doing things right.