It depends on the type - certainly though, any mutagen would not just produce a single viable mutant embryo - if it was producing mutations that still remained developmentally viable, there would be dozens to hundreds of deformed fish, in addition to the ones that never developed past an embryonic stage.

Well that now makes more sense. Certainly if in an area there are more mutations in general, what would be seen is an increase in several deformities, but not a lot of dead fish. At this point we cannot continue speculating really, because we do not know what the source of mutations would be, if it produces certain mutations with an increased probability or is completely random and how many less spectacularly deformed fish there are around in that area. A missing tooth or eye or smaller fin would probably not end up in the news.

That's also not rare as a mutation in any population, including humans, especially since it can be passed on to offspring. Mutations in human include everything from non-brown eye colours to increased height to people with extra fingers. Fairly recently, there was a kitten born with two heads (they were joined together and only one was functional), which got a bit of media coverage, and that isn't unheard of either, just extremely rare.

There's a level of understanding of genetics that is a prerequisite to informed commenting on mutations.

Well regarding mutations to be passed on, this of course only hapens if that mutation is not quite impeding. A third eye probably is not, but I'd not even know if it would be passed on during the reproduction at which time the genes of two people mix and problematic mutations can be dominant or not. Wit three eyed fish, my suspicion is that it will not persist (or there would be a lot more three eyed fish around).
The problem with making a connection of one single instance of an unusual mutation to a potential cause (e.g. radiation, toxic substances, heavy metals) is that there is no way to make that connection with certainty. A mutation has no label on it saying what caused it usually, though certain types of mutations are more likely caused by some reasons (e.g. lung cancer coming from things that can be inhaled). So the only tool science usually has to refer to is statistics, and to perform that, one needs a large data set. The "detection limit" so to speak is rather high. You'd probably need several dozens or even hundreds of unusual mutations connected to some locale or substance to make a claim for significance of the connection and causality.

Yeah, it sounds like everyone in the area got worked up after they heard about that one fish :hmm:

Definitely. That's a worrying indication of the level of education people have (although Argentina isn't exactly a perfect country), although at least the site in the OP managed to report without making implications.

Here's a two-headed kitten: Two-headed kitten has twice the cuteness of regular kittens | Metro.co.uk
With the population of lifeforms, strange mutations that create such features, while still extremely rare, do happen through simple probability. If the chance if something is 1 in 7 billion - an exceedingly rare chance far smaller than most non-unique mutations - then statistically, an average of 1 human will still exhibit it. Nobody condemns Australia (or Perth, or excessive heat, or the southern hemisphere) because of the aforementioned kitten :P

Going into the source of the article (infobae.com, not the best source of information around here), the fish is a tarira, local name for the Hoplias malabaricus, a fish that likes warm, slow flow, shallow muddy waters and that can breathe oxygen from the air if it needs to. It's a nocturnal predator and eats nearly everything (according to the Spanish page of Wikipedia). Given that there is no recurrence of similar features in other organisms in the zone, the most likely explanation for the third eye is that it just happened. If there is recurrence, then we can start to speculate about contamination. Genetics is just a lottery.