Aquarium fish water is rich in plant nutrients and nitrogen based compounds. They would make ideal fertilizer for household gardens. If you are an aquarium keeper, you can keep a small garden ( at least a few potted plants if you live in an apartment). This will help you utilize the waste water from your aquarium.
However, do not use the water if you have added medication or any pesticide for algae or for snails.
These pesticides can affect the plants. Hence, use the water only if you have not added any medication or chemical to your tank after the last water change.