Basic aquarium care includes periodically changing portions of the fish tank's water. Depending on the tank's size, contents, filter and other factors, water changes may be needed monthly, weekly or several times a week. In many cases the water removed from the tank can be applied to houseplants or garden plants rather than poured down the drain.
Water changes are performed to remove excess chemicals and other materials that could harm aquarium fish if allowed to build up. Those very materials are beneficial to plants.
Aquarium water accumulates nitrogen, phosphorus, potassium and ammonia, plus beneficial micro-organisms that process these materials. You may recognize these as ingredients in plant fertilizer and soil amendments.
Because it's so important to follow the timing and dosage instructions on packaged plant food, you may wonder if reusing aquarium water is safe or if it will result in an overdose of nutrients for your plants. Rest assured that if you're changing your fish-tank water regularly, it will be dilute enough to apply to your plants every time you water them.
But not all aquarium water can go on indoor or garden plants. Avoid using water from salt-water tanks, because the amount of salt may harm the plants, particularly those in pots. If you've used chemicals to adjust the water's pH, ammonia or other chemical levels or to treat your fish for diseases, do not apply the water to plants being grown for consumption. Very dirty aquarium water that has not been changed for a long period should also be kept off plants, as it may be too much of a good thing.
