Safe for use in both freshwater and saltwater tanks, this Penn Plax scrubber pad is designed to remove algae and hard water deposits from your glass tank or aquarium. Use this product with glass tanks only for best results and to avoid scratching. Available, but sold separately, are scrubber pads for plastic or acrylic tanks. This specific product measures 3 x 4 inches, but other size options are, as well, available (also sold separately).