Diatoms are unicellular algae that consist of many species that all have a cell wall made of silica. They are photosynthesizing algae, as are many other types of algae. Their yellowish-brown chloroplasts, the site of photosynthesis, are what gives them their golden brown color. Diatoms are found in the oceans, lakes, ponds, rivers, and even in damp soil and that mud puddle in your backyard. In other words, they are found anywhere there is water.
Brown diatom algae is a type of diatom that is often seen in new aquariums that have just completed their biological cycling process. The algae bloom may be just a light coating of brown on parts of the substrate, rocks, and aquarium walls, or it might be a complete coating. Brown diatoms are autotrophic and therefore are restricted to areas that contain some light in order to grow and reproduce. Brown diatom algae can be found in the oceans down to depths of about 600 feet.
To get rid of brown diatom algae from your saltwater aquarium, you can try siphoning the diatoms off of the substrate and rocks and brushing them off of the tank walls. You can also try adding snails to your aquarium as they feed on diatoms. A UV filter can also help to reduce diatoms