IMO, uronema and velvet are the quickest and deadliest that I have dealt with. Brook is deadly too, but not as quick, IME, followed by ich and flukes. Ich and flukes can be cured pretty quickly once identified. Once uronema and velvet are identify, it is usually too late for a cure. For uronema and brook I use Formalin and CP. For velvet it's CP. For ich it's TTM and for flukes it's Prazi-Pro.