If you have tropical fish you no longer able to keep, there are several options.
1) Tell all your friends and ask them if they would take in your fish. Throwing in the fish-keeping equipment, such as the tank, heater, filter, lighting and so on, may make them willing to do this if they are not otherwise able to.
2) Go to your local petstore and ask them to take back your fish. Some petstores will pay for returned fish, and more will accept free fish so long as they are healthy. I have used the previous two options exclusively in the past when I moved to a new city without a car. If, however, you really find yourself stuck:
3) Put an ad saying “free tropical fish” in the buy-and-sell.
4) Contact an aquarium club and offer your fish there.
5) Some SPCAs may take fish, but not all will have facilities for keeping them.
6) Feed them to your carnivorous pet. Warning: some fish are toxic. You may also not wish to do this to your pet fish.
7) If you really cannot find a home for them, I have been told that a humane way to kill small fish is to put them in a freezer with a very small amount of water, or for larger fish is to sever the backbone just behind the skull.
It is possible to move fish when moving house, or even when moving cities in some cases. As I do not have a car, I have never been able to try this. All the accounts I have found sound quite complicated as the fish must be kept at a reasonable temperature and with sufficient oxygen during the move. The larger the distance they must be transported, the more difficult this gets. Whatever you do, do not release them into the wild. For an explanation as to why they must not be released into the wild, please see the article on invasive species in the fish conservation section of this site.