That's true for many tanks, though it is possible to get rid of it completely with ample quarantine measures. I don't know how many actually go through all of the proper procedures, though (I don't, admittedly).
FWIW, flow rate isn't a deciding factor, as the parasites have more than enough capability to latch on. When I worked at an LFS, we had ~100x turnover and fish still got ich. On the other hand, it may indirectly help against it by oxygenating/exchanging CO2 and minimizing the associated stress.
FWIW, flow rate isn't a deciding factor, as the parasites have more than enough capability to latch on. When I worked at an LFS, we had ~100x turnover and fish still got ich. On the other hand, it may indirectly help against it by oxygenating/exchanging CO2 and minimizing the associated stress.