My Friend Ed has an aquarium which is a hobby I’m interested in as well. Perhaps once things settle down with the house I can start one. Ed told me something interesting, when he changes the water in his tank, he saves it and uses it to water his plants! My first thought was… hoarding old, dirty fish water is totally weird and gross but Ed explained that fish excrements in the water act as a natural fertilizer for plants*.
Ed has a stellar plant collection (see above), so it all makes sense. Google also concurs:
Before you pour your aquarium water down the sink, consider using it to water your plants. This not only saves water by recycling it, it saves you money on expensive fertilizers. Aquarium water is a rich source of nitrogen, which is what plants need to make chlorophyll, an essential component of photosynthesis. It also contains phosphorus, which supports healthy root growth, and potassium, which makes plants strong and helps them hold water. Used fish water also contains beneficial bacteria that break down ammonia and help feed the soil. Water from outdoor fish ponds can also be used to water your garden. Read more >
If you have a tank, consider getting some plants or if you have plants consider starting up an aquarium. Both will benefit!
* Not all fish tank water is appropriate for plants, however. Salt water from marine tanks cannot be used as fertilizer because the salt will kill the plants. Additionally, chemical pH adjusters or fish medications used in some aquariums are not safe to use on fruit and vegetable plants that you plan to eat. If you have neglected to change your aquarium water for a long time and it is very dirty and bad smelling, use it sparingly on plants or you risk overfeeding them. Sludge from aquarium and outdoor pond filters can also be fed to plants, but use it as you would fish emulsion or dilute it with water first. (source)