seachem fluorite black sand substrate, 7.7 ounces each (15.4 lbs) seachem 2 pack. seachem flourite black sand substrate. seachem flourite black sand substrate is ideal for natural planted aquariums.this sand is highly effective when used alone, but you can consider mixing it with gravels. there is no need for gravel modifiers such as laterite. it doesn't have any sort of chemical coating. the appearance of this sand is ideally suited for aquatic plants, but you can use for aquarium environment. product features: ideal for natural planted aquarium. this sand is effective when it is left alone. it does not have any chemical coating.