Diatom Algae & "New Tank Syndrome"

Key Symptom: A fine film (usually light brown) on substrate and surfaces that can be wiped or brushed away easily, may be a solid sheet or have a stringy, cotton-candy-like appearance. If you see a thick mat, usually red or green in color with bubbles underneath the mat, this is Cyanobacteria and NOT the Diatomaceous Algae discussed here.

A diatomaceous algae bloom is one of the most common events that aquarists will experience in both freshwater and saltwater aquariums. It is commonly referred to as “New Tank Syndrome” (NTS) since it is often seen in the first few weeks of the cycling process of a new tank or after a tank has gone through an event like a massive water change or medication that may have eliminated the beneficial bacteria population in the tank.

The Nitrogen Cycle

To understand NTS, we first need to understandthe cycling process of a new tank. When we refer to “cycling a tank”, we aren’t referring to the water circulating through the filter. We are referring to the chemical cycle that every tank will go through that stabilizes the water quality and allows beneficial bacteria in the tank to colonize and help reduce waste material. After the first organic material is added to a tank - usually fish in freshwater or saltwater, live rock in saltwater, or a material like fish food in a “fishless cycle” - bacteria will begin to break down the waste. First, the organic material (fish waste, decomposing fish food, etc) will begin to decay and produce Ammonia. A bacteria known as Nitrosomonas will break down the Ammonia into Nitrite, then another bacteria known as Nitrobacter will break down the Nitrite into Nitrate. These bacteria blooms are what cause the white cloudiness that aquarists see within the first few weeks of a new aquarium. After that second bloom, you may see a thin brownish film coating the substrate and surfaces in the tank; this is usually the diatomaceous algae film we call NTS. There is another filmy “algae” found in aquariums caused by cyanobacteria. This “slime algae” is often red or green and is much thicker in appearance than NTS.

Older tanks may also sometimes experience NTS blooms, usually after a very large water change (over about 50%), after a medication or change to the tank that kills off the bacteria population or a change in water source that may have added diatoms to the tank.

What is it?

The “algae” you see is not a type of plant with cells like traditional green algae that most people think of. It is actually made up of diatoms, a type of phytoplankton whose cell walls are primarily made up of the mineral Silica. It blooms towards the end of the cycling process in a tank because of the imbalance of nutrients in the system but will usually die off on its own once the water chemistry in the tank stabilizes. Once the water chemistry balances and there are no spikes in the Ammonia, Nitrite, Nitrate or other mineral levels, the diatoms lose their food source. Once the food source is gone, the bloom will go away on its own without adding tons of snails and hermit crabs or doing unnecessary water changes that will just slow down this process.


The best treatment is patience. Avoid the temptation to add unnecessary chemicals or do large water changes and give the tank a chance to stabilize itself. Depending on the size of the aquarium and on the bioload in it, the cycling process can take about 3-6 weeks to complete. You can add a bacterial supplement to help seed the population faster, but a new tank will eventually produce this all on its own with time.

Related Issues


Cyanobacteria (“cyano”) is commonly called “slime algae” and may be dark reddish-purple (especially in saltwater), bright green (especially in freshwater), black, brown or blue. It is actually a type of bacteria and is not algae at all although it does behave similarly. It usually grows in thick mats over surfaces with air bubbles underneath it, or sometime in stringier forms in high flow areas. It is usually caused by a water quality issue like high Phosphates, by poor light conditions like fluorescent bulbs over 6-8 months old or several other common causes. It is not commonly seen in a tank that has just been set up or reestablished recently like NTS.

Key Questions for Diagnosis:

1. Is this a new tank (under about 6-8 weeks old)?
2. Has the tank been “re-started” recently by an event like a major water change, filter change or medication?
3. Has the tank gone through a Nitrogen Cycle recently (spike in Ammonia, followed by a spike in Nitrite, followed by a spike in Nitrate)?
4. Is the algae bloom thin and filmy or thick and matlike?