milky algea

My guppies are dying? One at a time. About every third day and only the females? 2 year old, Fifteen gallon tank, 8 adult guppies, just had 25 fry. Also 2 Kory catfish, about 1-1/2 inches long. My fluval (20gl size) filter gummed up with a milky algea looking stuff and ran dry. I had no choice...