I have had HITH disease on my ex orino too. Goldfish are only half the story, the toxic matters in our tank are the problem.
My solution is simple; plant your tank with real plants. Any fast and easy grower can be had; check my photos. Even water lettuce help. Never had this problem any more once i have a planted monster tank. These living plants help remove toxic matters that are harmful to your fish and improve the water quality too. Hey, fish are like us, they love greeneries too for the same reasons!
PS. I don't do WC, only top up once the sump gets dangerously low.
