Yes, do not give up, it takes time. It is certain that some protozoan will get through our defenses, be it heat or whatever, so you have to be persistent.

It is now believed likely that ich is always present in most of our tanks, as it is in natural waters. There is really no other explanation for a sudden outbreak in established tanks that have had no new additions for over a year. Stress causes it, plain and simple. If the fish are healthy, they fight it off. When weakened by stress, they are more likely to succumb. It is most prevalent with new fish due to the extreme stress caused to fish being shipped, dumped in a store tank with probably less than ideal conditions, then chased around with a net, bagged, and carried home, then another new environment. Hence, we QT so it is easier to control and eradicate.