no injections are present. the hypo stands for hypo-salinity, not hypodermic needle. Hyposalinity meants to take out salt so the salinity levels go down. Fish can adjust to these changes, as long as it is not all at once, but many other types of creatures such as corals, live rock, and many parasites like ick cannot. This is why it is used in a tank with just the fish. This is why the water doesnt need to get into the fish, because parasites like Ick are external, so they are killed jsut by being in contact with the water. With less salt, the pressure of the water is much different, and can be devistating to smaller creatures, esspecualy those made up of mostly water (like anenomes). If your eel does have an internal parasite like some type of flat worm, i dont think this would be effective.
to JDM; u are very right about the mucus, it makes it so that the parasite isnt able to hold on. But all fish are like this under normal conditions with their "slime coat". But it is when a fish become stressed, due to poor water conditions, malnutrition, or even from another fish picking at it, that ick becomes present. I am pretty sure an eel can get ick, but it would probably be much harder. snowflakes are also very hardy too, so it would be rare, but im sure it could happen.