Why Would Flukes Re-Occur After Treatment?
I've heard it said that Flukes can "just appear" in a closed system, spontaneously. If you clear them, and prove they're gone with your microscope, they stay gone.
Costia - The Big Long Article
Costia is the smallest but perhaps the most deadly parasite. Hard to see under the scope, and harder to kill. But here's the long and short of it.
Chilodonella - Deadly, But Easy to Clear Ciliate Parasite
"Killer Don" as Jack Gratzek calls him - this deadly ciliate is common in the Spring and can kill a mess of fish fast. Easily cleared with salt.
Costia (Ichthyobodo necatrix) Overview
Costia is a quick killer in Springtime and is a small, comma-shaped organism under the microscope. Little else can kill fish so fast.