BTW - some may find this interesting - (I'm not going to use the scientific terms for the different CI stages - because it gets confusing for everyone). But - CI (Ich - Cryptocaryon irritans) - is actually interesting.
A particle infects a fish - and eventually generates the 'white spot' - after growing a bit. This is then released from the fish - (and as it's on the fish - probably gives at least SOME immunity to that fish). This occurs usually in the early AM hours (like after 2AM). Fish tend to sleep in the same spot every night - Thus - the particles are released from the fish - and the secondary particles - fall to the area near there. They form cysts - which can last for days (not going to get into how many days) - but days - during which time they are making more infectious particles perhaps hundreds per original particle. Then when the fish comes back - they are re-infected - this time with more.
The moral of the story - and this is my personal observation/opinion - FLOW - in the tank is important. It takes the particles away from the original release point - and when the now highly multiplied infectious particles are re-released - they are going to be passed far away from the fish.
Stocking density (again IMHO) - is a big reason why some tanks have 'no problems with 'ich management'' - whereas others do. The more fish - the more surface area for infection - and thus the more infections (they are Theronts OK OK I did it) - that are going to released near their prior fish host.
I clownfish in a 500gallon tank with CI - is not likely to be re-infected. 5000 clowns in a 500 gallon tank with Ci is likely to be a disaster.
My editorial comments