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In Nature:
A tank is a closed, unnatural environment where parasites can quickly reach numbers that can harm or kill a fish. I can tell you from experience that gill flukes are a very successful parasite and can easily cause the death of many discus Fry. Hexamita, a protozoan parasite, along with Capillaria, a nematodal parasite, are probably responsible for more discus deaths than any other factor except water quality in a hobbyist tank (in my opinion).
Tapeworms, by their life cycle, usuallly will not cause a direct death, but they open the fish to infection at the site of where their mouths attach. They compete with the fish for nutrients and take up "space" decreasing the useable digestive area (Imagine how much space six each 6 - 11 inch worms would take up in the intestines of a medium size discus.)
One other note to consider as well. The parasite-host relationship really doesn't deal with individuals. It's based on populations. In many cases, the host is killed either directly or indirectly (infections, nutritional, etc.). But the relationship is deemed a successful one from the parasites' point of view as long as the death of the host comes slow enough to allow for the parasite to reproduce sufficiently to allow its species to survive. In the case of the tapeworm, that would be long enough for the worm to reproduce by its gravid egg-filled Proglottids breaking off and being passed with feces out for another host to inadvertently ingest and perpetuate the life cycle.
 
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