Mature matters most when you're talking about waste disposal and resiliency of that system. It does help with water stability, but water stability also helps with a tank's maturity, as unstable tanks rarely last long. Ich is not a magical condition that appears whenever fish are 'stressed.' It is a bug, a parasite, a pest like fleas, and if you create conditions that favor one more more ich parasites finding a fish whose slime coat isn't good, or conditions in which your tank water is ich-soup, you'll almost certainly have a problem. The same way one or two fleas finding their way into a house with no dog or cat is not generally a problem to the other inhabitants, but if you have two fleas in a house with a sickly dog or cat, it can get very, very bad. The parasite needs a host, and your job is not to provide it one.
