Have you noticed a golden-brown film spreading through your tank? Have the walls of your tank becoming dingy with what looks like a green-brown film growing on the inside? You may be a few steps away from having a diatom problem, which, if left unchecked, can destroy the water quality in your aquarium and kill your pets in a very short time.
Many species of these kinds of algae exist in your aquarium and can harm your fish merely by blocking light and monopolizing space. Should they become too prolific, the dead and decaying population will quickly increase toxin levels and reduce the amount of dissolved oxygen available for your fish and invertebrates.
Diatoms are a type of golden-brown algae (Chrysophyta) that secrete silica - or “glass” - in order to form their microscopic homes called frustules. Some scientists classify diatoms in a group all to themselves, the Bacillariophyta. As their nickname “grass of the sea” implies, these phytoplankton are ubiquitous and thus will appear at some point in just about every well-lit, well-run saltwater aquarium. These plankton are often the first to colonize new surfaces in both fresh and saltwater tanks. In some cases, diatoms are the result of problems with your initial set-up and will subsequently cause difficulties with maintaining good water quality throughout the life of your tank.
Encrusting diatoms are most dangerous in reef tanks since they will first start to grow at the base of corals and move upward. They will cause the corals to recede and in a matter of a few days can kill the colony.
When You See a Buildup
If you begin to see a buildup, which becomes obvious as the diatoms adhere to the tank walls giving them a brownish tinge, clean the panes with a soft cloth or sponge. Begin at the bottom and swipe slowly upward in one sweep. If you keep surgeonfish, tangs or damselfish, they will graze on the loose scrapings. Clean the sponge after each sweep. When the all the panes in your tank have been cleaned, allow some time for the particles in the water column to settle. Then, siphon the gravel and substrate for the loose particles and dead and dying algae since these provide a constant source of silica, and decomposing will reduce the water quality in your tank. Siphon and clean the tank weekly until you notice a reduction in algae growth.
The best way to deal with a diatom problem is prevention. In order to produce their frustule and live out their lifecycle, diatoms need abundant silicates or silicic acid. Silica sand is a common source, so it is highly recommended that you use a limestone sand or gravel instead. Some commercial salts for mixing marine water will also increase the levels of silicates - be sure to read the labels and know what you are adding to your water.
Water also contains silicic acid, so treat the water with agents designed to reduce the acid to at least 0.5 ppm, since you can never eliminate all of it. Silicate-reducing compounds and specialized filters are readily available from commercial aquarium supply stores. The material used in these filters must be changed more frequently than your regular filter, depending on the size of your tank, the water quality and the consistency of the diatom problem. Check the manufacturer's recommendations for the best results.
When you perform water exchanges, you can reduce the amount of available silicates by filtering the “new” water three to five times through filter sheets that contain compounds that will eliminate silicates. This is especially wise to do if you want to eliminate a persistent problem with silicate levels. As you keep treating the water you will find that diatom growth is slowed to “normal” levels. Test-kits for silicates are available although not really necessary if you keep an eye on your tank for a potential problem. If you have waited too long before taking measures to keep the silicate levels low, you may even have to double the frequency of your water exchanges for a couple of weeks to solve the problem.
Keeping grazing invertebrates such as snails is also a good preventative measure because these species will roam the tank and graze on the algae both on the tank walls and in the rocks, crevices and other hard-to-reach spots in your aquarium.