I use this occasionally when I see sludge in the bottom of the tanks, or when the gravel becomes discolored. I will use this at night and let it stay overnight. It doesn't harm the fish or any of my plants. However, it is black and you will barely be able to see your fish after adding this product. The fish seem to be unaffected by the stained water. After a large water change, the tank looks sparkling clean, and any sludge or discoloration of the gravel has disappeared.