Fish often have only a few parasites when you buy them, but when put into this condition(small tank, water quality issues), the parasite population will exponentially grow. Every 4-7 days, ich population can multiply by 256. So in two weeks, for each single parasite the fish brought in, you now have a potential of 65,536-16,777,216 parasite babies feeding on your fish.
Of course not every parasite baby survives to eat the fish, and a healthy fish has an immune response to kill more. But a small tank means the parasite babies don't die in the empty open ocean, they will mostly all hit their target, and with bad water quality, the fish's immune response is lowered.
-Matt
