What do you do with the leftover water from your fish tank maintenance?
I use it to water my plants. It makes a nice no-burn fertilizer for my herbs. I used to use fish emulsion, but since I became a vegetarian, this has been a humane solution that costs nothing!
This is a page about using aquarium water for plants. If you have an aquarium consider using the waste water when you clean the tank to fertilizer your garden or houseplants.