Ummm... I think its pretty well known that the parasite is the "ich", and your fish cant catch ich unless the parasite already exists in the system or is introduced.
From what I have read and heard from others, fish have a resistance to the parasite (this is over-simplified Im sure). If a fish is living in unsatisfactory conditions (poor water, harassment by tankmates, malnutrition, etc) or has adjacent health issues, its resistance may be weakened.
My experience has been consistent with this thinking. I accept the fact that ich exists in my system, but I also believe fish should be resistant to it normally. If a fish is affected by the ich, I know that something is wrong: a) with the fish's health, b) with the fish's environment, or c) with the fish's diet. These are very broad conditions of course, which can be even further encapsulated in the broader term "STRESS". Any of the things mentioned above constitute stress, per my perception anyway.
Personally, I have never witnessed any of my own fish spontaneously contract ich. There was always something that triggered it, and I have been fortunate enough to be able to correct it every time (about four cases total). Here are a couple of my ich experiences.
I had two yellow coris wrasse for approximately six months. One somehow damaged his left fin quite badly, and it became infected. The next day, he also was affected by ich. The second yellow coris was never affected. I was able to get rid of the ich four days later, but the fish did perish as a result of the fin infection despite my efforts.
In this same tank about a month later, there was a power outage and the tank dropped nine degrees one night, to the low 70's. The next morning, all fish (a six-line and two neon gobies) had spots. I slowly corrected the temperature, fed heavily several times a day with garlic supplemented food, and all fish returned to normal by the second day (third for one of the gobies).
I battled my next case of ich by choice. While in a local store, I did something I knew better than to do and purchased a tiny sailfin tang that was covered with spots and lying on his side. He looked physically healthy, clear eyes and good color, other than an obvious case of malnutrition. I was heartbroken, and something told me I could nurse this little guy back to health. Well I did! Im ecstatic about it. I brought him home last sunday, started feeding a varied diet four times a day (formula 1/2, primereef, minced krill, brine shrimp) and made sure he had selcon-soaked nori and caulerpa available at all times. Getting him to eat the first day was tough, but he took to the nori quickly and then things got easier. By Wednesday, the third day I had him, he was clear of all ich and swimming quite happily. He's also got one hell of a gut!
So, while none of us may know what causes ich, I have my theories. Help the fish gain his resistance back and he will do the rest. This is all just my opinion, of course.
