My mom and dad have recently built an outdoor pond (3 weeks ago) We have 3 fish in it, a few days ago I noticed that most of them starting acting very sluggish.(I figured it was just because of the colder weather), then a few started developing this white filmy looking stuff on them. Today 2 of them have died. I can't tell if the others have the stuff on them b/c they stay close to the bottom. Does anyone have any idea what this could be, and what I could do to save the remaining fish.