What is this white stuff?

[Shana]

I have an aquarium that has been up for a little over two months. It was cycled with only plants living in it, fish were added about two weeks ago. Yesterday I did a 50% water change and about an hour later I noticed some whitish "stuff" all over the interior of the tank. I thought it was air bubbles from the new water and ignored it. When I got home today it was still there, so I touched it to find out what it was. It is not air. It seems to be some kind of slimy film, and it is coating everything in the tank.

I did some water tests:

Ammonia: almost .25
Nitrite: 0
Nitrate: almost 5
pH: less than 6

These seem odd to me. I do not know how the ammonia and nitrates can be so high after a water change (granted, I did not test the water immediately before the change) and the pH is very strange. I live in Chicago, and we have rock hard water. Lowering pH is a huge challenge, so I have no idea how it got so soft.

I took a picture, but can't seem to attach it in any form that will actually post on here. This might be due to my slow internet, I don't know.

What do you think the white slime is? I can't think of anything that would spring up out of nowhere within an hour of a water change.