In most cases, the fish can survive with them. Other times, the fish becomes sick and dies. Because this is a huge category in fish keeping, I do not have time to add more information here at this time. I hope to add a chart of fish diseases and cures soon, maybe on its own page. Parasites Large parasites include anchor worm, fish lice, and fish leeches.
Small parasites are often protozoans.
Most medications against parasites are poisons. This means that sensitive fish like many catfish, discus, etc. may die from the medicine. In soft water especially, these medications are more deadly. Always try a new parasite killer first at half the recommended dose to see if the fish can take it. Even then, they could still die.