Finding plants
If you collect plants from local swamps or trade them with friends, buy from an unfamiliar plant store, please quarantine those plants for a few weeks. Put them in a washtub or kiddie pool with a bit of chlorox in the water.
Parasites, lice, fish eggs, caterpillars and so many other critters can be carried in on those plants and play havoc with your goldfish, your beloved koi and all your aquatic plants.