Parasites: protists

Protozoan parasites. For some beginner fishkeepers, "parasite" is almost a synonym for "invertebrate." Not for you, though. You've penetrated this far, so I'm sure you're perfectly aware that if you see a creature living free in the aquarium, no matter how undesirable you may find it (call that the "Eew Factor") — it's not a parasite. If it's an invertebrate that we admire and respect, you may be able to find out more about it among "Invertebrates" in the menu at the left. Alas, we treat other invertebrates callously as merely temporary members of the community — they get no respect ...because they're "Live Food!"
 
The third category of invertebrates, the badboys in the pages that follow, are all parasites: First, in this "Parasites: Protists" subfolder, you'll find articles on the most common parasitic single-cell protists; then, in the "Parasites: Invertebrates" subfolder that follows, a few of the most common multicellular invertebrates that parasitize fish: flatworms, which include flukes and tapeworms, copepods — which are minute crustaceans — and a nasty intestinal nematode worm or two. Not a complete roster of fish parasites by any means, but if you get some understanding of just a few parasites, you'll have the practicalities of the situation pretty much under control.
 
Let me remind you again that I'm not a veterinarian. I'm just passing on to you the best of what I've read. Don't rely entirely on hobbyists such as the Skeptical Aquarist: probe the information that biologists offer to veterinarians and aquaculturists. If you need more information, or your fishes' problem isn't treated among the few common parasites here, you need to explore some of the links to fish diseases on the Web. Begin with some of the wide-ranging sites mentioned in the section Ich on the Web.
 
Parasites in general. Parasites have traded the chance of an easy life for steep odds against finding a suitable host. In the course of evolutionary time, intestinal parasites like some flatworms or nematodes, which are constantly bathed in absorbable nutrients, can dispense with a digestive system altogether. Instead, the parasite concentrates its energy on producing eggs or mobile young. The chances of finding a suitable host are so slim that the female of a common nematode of humans and horses, Ascaris, produces 200,000 eggs — not in a lifetime, but in a day.
 
The crowded conditions of even a well-kept aquarium multiply the chances of locating a suitable host by several hundredfold. So parasite loads that could be sustained under natural conditions can multiply and overwhelm the host fish in an aquarium.
 
Virulence is not in the parasite's own long-range best interests. All parasites have co-evolved with their natural hosts to a kind of cold-war truce in the race to evolve new defenses. A dead host may be the end of the line for most of its parasites too. In the narrower ecosystem that an aquarium defines, fishkeepers themselves become a new Force of Nature! If at first view, this seems like an extravagant statement, consider it from this angle: a minor, relatively harmless ciliate like Tetrahymena may thrive unharmed through many generations, whereas a more destructive one like Ichthyophthirius — once in captivity — provokes destructive changes in its environment, unleashed by a furious aquarist.