I have a Fluval Flex 15-gallon tank that came with filter media when I started it. Initially, there was ammonia in the water, which prevented me from getting any fish. So, my LFS suggested I get some bio balls, which turned out to be helpful. I haven't cleaned them since I first got them a few months ago. But recently, I got my water tested, and it was ammonia-free. Now, I'm not sure if I should keep these bio balls because some people claim they can be a nitrate factory.