Maybe that's what it is. I also cover it with a lid to prevent dust and other nastiness from mixing in. I think I'm going to change the way I do water changes though. I typically do about 30G every month. The problem with this is 1) stability and possible drastic parameter changes and 2) it becomes this epic ordeal. After reading Ace's 5G twice a week post, I think I'm going to do the same. It's easier each time and would not change water parameters since it's such small amounts each time. I would really like to figure out a way to automate the whole process.