My list of fish that should not be sold include:
1) Most fish that grow to more than 1 foot in length. Some of the smaller eels (3 foot long ribbon eel) are ok because they aren't active swimmers and normally reside in a smallish territory that can be accomodated in an aquarium. But things like redtail catfish, arowana, arapaima, black pacu, etc, simply get too big for aquariums and literally need huge ponds to live in.
2) Any fish that grows big by aquarium fish standards (8 inches plus), needs to be kept in a group, and is an active swimmer. Silver sharks and tinfoil barbs fall into this category. They can reach a foot long, need company and need room to move. Very few people have a tank big enough to house 20 of these fish when they are mature.
3) Fish that are so inbred their bodies have been significantly mutated and no longer resemble their original wild form. These fish regularly have internal problems and can't swim properly. All the balloon fishes fall into this category, along with the short bodied goldfish.
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BETTAS
Male Betta splendens are highly territorial fish that only accept female Betta splendens into their territory when they want to breed. These fish are solitary by nature so having one on its own in an 18 inch long tank (or bigger) is fine and helps to replicate the fish's natural lifestyle. Having them in tiny cups or little tanks with 1/2 a litre of water is not good for them but is the only practicle way shops can hold large numbers for sale.
The exceptionally long fins on some male B. splendens, and the inbreeding of this fish is a major concern and has weakened the fish we see in shops. The inbreeding needs to stop, and the fish should be able to move through the water without too much effort.
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GLOFISH
GloFish are made by injecting jellyfish genes into a developing egg. Once the fish has the gene, they have it forever and pass it on to their offspring. In my opinion, it is less stressful and more humane to the fish being sold because their ancestor was injected (genetically modified) as an egg but none of the offspring have been touched. Comparing this to dyed, painted or injected fish, that literally get man handled, painted or injected with a syringe, kept out of water for 30-60 seconds so the paint can dry, and then put back in the water. These dyes and paints come off over a period of time or the scales fall out and the fish ends up being normal coloured after 6 months, assuming they live that long.
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OSCARS
Oscars aren't a bad aquarium fish because they are an ambush predator that has a territory and isn't an active swimmer. This makes them suitable candidates for a good sized tank (4x2x2ft or bigger).
Some of the other big cichlids and perch also fall into this category.