This is a topic that comes up repeatedly and even if WE here at MFK were able to agree on a solution, there is no real point since MFK can not enforce anything. There simply is no point in exerting a lot of effort to convince everyone what is best unfortunately. The only hope threads like these have is to help educate some people. With this in mind, I propose an "academic" discussion since the reality is quite tragic. Obviously we need to consider this from all sides, including the fish collectors, fish farms, distributors, retailers and consumers.
There is one solution which people do not seem to consider which I feel is quite effective and relatively easy to implement. The simplest and most effective solution IMO is to simply raise the price of problematic fish. Increasing the price at entry point to a country is the easiest to implement and simplest to police. Example - any invoice which shows certain species will attract a basic surcharge. The amount of surcharge will depend on the severity of problem fish (species) but several hundred dollars was what I had in mind. This surcharge like any other charge will be paid by the importer who will naturally pass the cost on down to the retailer who will then charge the customer accordingly. This immediately forces retail shops to consider their market much more carefully and may encourage custom ordering. Since the fish has a higher price tag, some shops will make the same margins with much less stock (reduced numbers) so shops will be willing to accept this scheme. Customers who pay several hundred dollars for a fish are much more likely to plan purchases more carefully (less impulse buying) and also house larger fish more appropriately. Naturally there will still be some illicit transportation, re-labelling of invoices, smuggling etc but despite this, the end result will still be a much higher market price for problem fish. If someone went to the trouble of avoiding the surcharge via illegal means, why would they sell it for several dollars instead of several hundred?. Most importantly, the incidence of dumping fish will dramatically decrease and the chance or re-homing unwanted fish will increase. Also quite importantly, this scheme allows serious hobbyists to continue buying and keeping the fish they really want.
I am not basing my ideas on pure fantasy but from many years in the commercial and international aquarium industry. The price of problematic fish such as RTC, arowana, Pacu, etc in Australia is usually around $500 each yet there are many people willing to pay these prices. This seems to solve any problems re-homing large unwanted fish, dumping in waterways, impulse buying, not upgrading tank sizes appropriately. The only losers in this scenario are the fish farmers since the numbers of these fish they sell will dramatically plummet. Hopefully these surchages can be used for consumer education, surcharge policing etc. For those of you currently paying a mere few dollars each for such problem fish species, this post may help you re-evaluate priorities.
I welcome other thoughts and different opinions but I am not prepared to get into any heated discussion since there simply is no point.