As long as you know it has been done, what really is the problem?
That is the problem. A lot of people hybridise fish and other animals and don't tell the world. They say new fish and everyone says "Yay, it's more colourful and adds to the collection of fishes we have in our aquariums. So why not keep it?"
If everyone was honest enough to say, yes it's a hybrid between X & Y and they are sterile and will not survive in the wild, then I wouldn't have a problem. Unfortunately most hybrids are not well known (eg: Siamese fighting fish sold in shops are a hybrid between Betta splendens and B. imbellis) and most hybrid fishes produce fertile young.
This can create problems if the fish are ever released into the wild and whilst most people don't release fish into the wild, it still happens. We have regulations in Western Australia where aquaculture ponds are not allowed to be located within 500 meters of a natural water way and yet fish still get washed out of their ponds and end up in local rivers.
People deliberately release fish into rivers too. We have some lovely Geophagus brasiliensis living in the fresh water creeks and streams north of Perth in WA and they were put there by an aquarist who wanted to have a local population to harvest and sell to shops. Obviously these fish are not hybrids and won't breed with native fishes but they have eaten most of the native fishes in the area.
We had an issue some time back in Australia where an alleged new bloodline of Glossolepis incisus had been smuggled into the country. A lot of fish breeders including myself bought some of these fish and mixed them in with our current stock to add new genetic material. We are not allowed to bring rainbowfish into Australia so our current stocks had not seen any new bloodlines since 1982 when quarantine regulations came into force and banned the importation of rainbowfishes. It turned out the new bloodline was simply a cross/ hybrid between Glossolepis incisus and G. wanamensis (G. wanamensis is on the endangered species list). Thousands of G. incisus were bred and put onto the market and 12 months later it was revealed the fish were in fact hybrids. The anal fin on G. wanamensis is massive whereas G. incisus have a small anal fin. Some of the young male G. incisus started producing huge anal fins and showed yellow and blue scales on their body. Because of this, thousands of hybrids were put on the market and sold as a pure species but were in fact hybrids. This screwed up bloodlines all over the country and put the true species at risk of becoming extinct in this country. It also ruined a number of fish farmers who got a bad name for selling hybrids even tho they didn't know they were hybrids until it was too late.
Most wild fishes are endangered regardless of what governments around the world say. The only reason fish haven't disappeared from the oceans and rivers is because of the number of eggs they produce. If fish only produced 10 eggs per year there would be no fish. Fortunately most fish species produce hundreds (or more) of eggs each year and that helps keep populations high enough to make people think there isn't a problem.
If we encourage hybrids and the wild fish stocks do die off, do we put hybrids back in the rivers and oceans?
If hybrids are released or escape into the wild and breed with wild forms, what happens to the original wild species? Is it acceptable for that to happen?
If people want hybrids, that is up to them. As long as they inform people who get those fishes that they are hybrids, and they don't let any get into the natural water ways, then it's not an issue. But not everyone is honest or can be counted on to do that.
