The first and third pics make it look worse than it was, it's reflecting the flash, but the second one shows about how much there was. It seemed to only be on the side that I open and close, and only on the top edge of the plastic. But it looked like cotton or something. I just wiped around the edge with a damp cloth, and did about a 1/2 water change.
I tested for ammonia (zero reading) and nitrates (very low reading) after I changed the water. I meant to do it before I changed the water but I have a short attention span and forgot xP
The only thing that I might have done different last night was that I gave Ralph some cooked carrots...
Has anyone else had some sort of mould pop up over night like this?
Should I do a complete water change?

