Water from your freshwater fish tank can be beneficial to houseplants. According to PetQnA.com, the nitrifying bacteria in your tank's filtration system turns the fish waste into nutrients that houseplants need. It's also a good way to recycle water. If you use more than a 5 percent conditioning salt ratio, the water might be too salty for your plants, warns PetQnA.com.
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Aileen Clarkson has been an award-winning editor and reporter for more than 20 years, earning three awards from the Society of Professional Journalists. She has worked for several newspapers, including "The Washington Post" and "The Charlotte Observer." Clarkson earned a Bachelor of Arts in journalism from the University of Florida.