In a healthy aquarium, particularly that of a community tank where you have several healthy well-tended fish, you can use the dirty aquarium water from water changes and your gravel vacuuming on your plants.
With the warm months coming in its free fertilizer compliments of your happy little fishies. A little thank you for all your hard work and a little bit of ease on the wastewater bill as its technically recycling. It can be used on food plants, house plants, flower gardens the works. As far as the plants are concerned your fish poo the plant equivalent of gold.
Wouldn't be a good idea to use water where you are treating for illness for some illnesses (such as tuberculosis) are human transferable and the holistic treatments of the salts can mess up/kill many land plants.
Thought maybe the gardeners on the site might like to know about that little gift the fishies give if they didn't already XP. Pet fish poo in thanks so your flower gardens may look pretty and your tomatoes yummy.