Laterite is both a soil and a rock type rich in iron and aluminium and is commonly considered to have formed in hot and wet tropical areas. Nearly all laterites are of rusty-red coloration, because of high iron oxide content.
Laterite creates improved growing conditions for rooted aquatic plants kept in aquarium gravel. It is a type of ancient clay found on geographically old continents. Younger clays may also be useful, but the aging undergone by laterite removes excess phosphate that can cause undesirable algal blooms in your fish tank