LeoR,
Having studied Cryptocaryon irritans, stress in fish, and fish nutrition extensively I have to disagree with you. I am not disagreeing with the importance of good nutrition, pristine water quality, or a low stress environment. All of those things do contribute to the overall health of the fish. What I do disagree with is the idea that properly taking care of the needs of the fish makes them immune to an external parasite, specifically Cryptocaryon irritans. Ich can and does attack otherwise healthy fish in an aquarium and in the wild. Wild caught fish that appear to be in perfect health often harbor ich. This is easy enough to prove by simply examining a scraping from their skin and gills. Even though the cysts may not be visible to the naked eye they still show up in many (not all) fish that are examined at capture. In the confines of a glass box there is no place for the fish to escape or simply swim aware to another area. This is why ich is not much of a problem in the wild, but a huge problem in aquaria.
We have to distinguish between a facultative and an obligate pathogen here when making broad sweeping statements such as “Proper nutrition makes fish healthy and provides all the stuff it needs for self-defense.” Facultative pathogens are those that are opportunistic and are only really problematic when the fish has a weakened immune system. Obligate pathogens are quite different and can attack perfectly healthy fish. Ich is an obligate parasite. There are three factors that must be considered and these are sort of like the three legs on a stool. Break any one of the legs and the stool will fall. First we must consider the health of the fish, then the quality of the environment and after that the presence and population density of the particular pathogen. These three “legs” if you will are more of a factor when dealing with facultative pathogens, much less so with obligate ones. With an obligate pathogen the fish can be in great health, have a quality environment and still become infected. All it really takes is the introduction of the parasite and a suitable host so that the parasite can multiply in numbers. One the population of parasites within the system has reached critical mass even the healthiest of fish can succumb. You see its not all just black or white, because pathogens are not simple organisms to understand and they do not all behave in the same manner. Its much more complex than I can explain in one short post.
Terry B
