Tinfoil barbs 6 for $2000. Tank included.
Make up labels for your tanks that include the adult size of each fish, shoaling data ie how many should be kept together, and minimum tank size to keep it properly. Appropriate tankmates. Aggressiveness. Warnings about potential problems.
Put a male betta in every tank that has peaceful community fish. Do not stock ANY fishbowls or betta houses unless they are at least 5 gallons and FILTERED!
When a mother and child come in for their fish, explain to the child how the fish must be taken care of if it is to stay alive and healthy. Such things as breathing and space to run around in will get the child relating to the needs of the fish they are buying. If the parent insists on taking the comets anyway, puts them in a fishbowl after you have explained they need a 20 gallon tank, and they die at least that is a lesson learned. That kid maybe will think through requirements for the animal in the future. Unfortunately, the parent is probably a lost cause. If someone has not developed compassion for living creatures by the time they are grown up and raising their own family, they will probably never develop that trait.