Live fish all have parasites. What you do by raising them at home in a controlled setting will keep the parasite load to a minimum. That's what you want as opposed to pet store, big box store aquarium fish. Their fish live in overcrowded aquariums, less than clean water, and poor filtration systems, all of which contribute to HIGH parasite loads which will get the reptile in trouble. Snakes thrive with small numbers of parasites, Protozoa, and bacteria. When those numbers increase and or multiply and become HIGH is when the animals get sick.