Well, I am in no position to diagnose so I hope someone else is. In the meantime while you are getting it figured out, maybe reduce the nutrients in your tank by withholding food inputs, and add a teaspoon of aquarium salt per five gallons of water. (Don't put the dry salt in the tank. Take out a gallon of water, dissolve the salt in the gallon, and sloooooooooooowly dribble it back into the tank where the water movement is greatest so that it will get diluted and spread around fast.)
Bump: You may want to look up "Oomycetes" too.