from what i've read long ago .. siapin is correct. in addition, treatments are only effective in a specific time frame of their lifecycle. before the ick is ready to attached itself to a host, it's vulnerable to treatment. ich digs itself underneath the fish skin, using the fish for protection.
i fought ich many time and each time using different strategy. i've lost 2 power blue and 2 sohal in the process. now im using cleaner wrasse, cleaner shrimps and mixing garlic extract with fish food. also, i stopped messing around inside the tank, stirring things up, and minimize submersing my hand and arm.
so far, i have both power blue and sohal in my tank for more than 9 months now - ich free. i just need them to stop slicing each other up