A single eye is usually due to injury, both eyes is bacterial. I use epsom salt (1 tblspn per 10 gallons, 1 tblspn per 5 gallons for fish only tank) which often clears it up. Don't know why it helps sometimes since epsom salt is just magnesium sulfate. It's just a one time dose so it either works or it won't (i.e. don't keep adding more epsom salt).