I think it’s done with bleach on black/dark-blue

Don’t quote me on that, but as far as I understand it, the scientists needed a control group for some experiment in breeding one of those species of fish, so they paired it with a different species, so far removed evolutionarily that the scientists believed there’s 0% chance of hybridization. Like there’s 0% chance of getting a hybrid of a platypus and a polar bear, for example. But then, against all odds and expectations, these two species of fish have produced a hybrid.