Which parasites are most prevalent in tropical aquarium fish? My article Parasite Surveys of Aquarium Fish, a compilation of 11 different investigations from around the world, answers that question. Freshwater fish (goldfish, guppies, tetras, etc) were sampled from Sri Lankan fish farms, Swedish pet shops, etc. Most surveys reported Monogeneans (i.e., flukes) as the most common parasite of aquarium fish–whether healthy or diseased. For example, 37% of 223 freshly caught wild fish from the Amazon carried Monogeneans, ranging from a 92% prevalence in Angelfish to only 7.4% in Cardinal Tetras. The ICH parasite was in second place (21% prevalence in the 223 fish), but other studies reported a much lower prevalence of the ICH parasite. Overall, Monogeneans dominated.
Diana Walstad's Books and Articles 