Hmm well from what I can tell it most definately sounds like eye flukes,also known as Diplostomum : a parasitic Digenetic Fluke (phylum Platyhelminthes) whose larval stage resides in the eyes of fish, causing the disease commonly known as eye fluke.
The most easiest way this mightve infected your fish is if, either A: your fish were caught in the wild, they then would have alredy had the flukes, or B: from teh snails in your tank, in the snail,larval flukes transform into a reproductive stage and multiply, the snail eventually sheds the flukes, which then swim out into the water in search of a fish host.
once the flukes get into the eye, usually via the gills and then the blood which carrys them to the small capillaries within the fishes eye, they grow to full size within 4 - 5 weeks and can remain in the lens of the eye for up to 4 years!!
hundreds of them at once have been found in a single fishes eye.
because in nature the the lifecycle is complete only when the fish is eaten by a suitable bird host, to then restart the cycle again.
In most cases, the fish usually stays healthy, and dont need to be kept in isolation, euthanasia is nessessary only if the fishes vision is severely impaired and is unable to locate food and is suffering as a result.
fish usually are able to locate food without relying on their vision as their first sense however, as well as other senses allowing them to swim without banging into other fish.
I hope this info has helped if your fish has eye fluke.
however if they have something else like pop-eye, which can also cause skin ulcers and such, then you may consider termination , as the fish will suffer.
goodluck.
Starry^