My daughter has had a Beta in a small tank for over a year. I started a 30 gallon tank 2 months ago and once it was established with a few Tetras and Barbs, we decided to add the Beta. He did great for several weeks but yesterday we noticed a white cottony-like film growing over his gills and this morning he was dead. I took him out of the big tank last night and took some of the tank water and put him back in his little tank, but now I am worried about the rest of the fish. Do I need to treat the water so the rest do not die? How do I figure out what the water needs?