Diatoms in the aquarium: causes and how to remove them
Description
Diatoms are unicellular microalgae that form a brown or golden film on aquarium glass, substrate, rocks, decorations and plant leaves. They can usually be removed easily with an algae scraper, a soft brush or a siphon.
They are especially common during the early stages of aquarium maturation. Diatoms need dissolved silicon to build their mineral cell walls, so their growth may be encouraged by the source water, certain substrates or some decorative materials.
They are not normally dangerous to fish or invertebrates. However, a heavy coating can make the aquarium look dirty and reduce the light reaching plant leaves. Their presence does not necessarily indicate poor water quality: the important question is whether they decline as the aquarium matures or whether a continuing source is sustaining their growth.
Why they appear
New or biologically immature aquarium
During the first few weeks, bacterial populations, plants and other microorganisms competing for resources are still becoming established. Newly submerged surfaces provide space for biofilm development, and diatoms are often among the first visible organisms to colonise them.
In many aquariums they gradually decline as the system stabilises, although they may persist when there is a continuous supply of silicates or organic matter.
Tratamientos
Manual removal and stable maturation
- Remove the film from the glass and decorations during water changes.
- Gently siphon affected areas of the substrate.
- Keep lighting, filtration and water parameters stable.
- Make sure the nitrogen cycle is progressing correctly before increasing livestock.
- Do not replace or excessively clean the biological filter media.
There is usually no need to restart the aquarium or use algaecides. In a new aquarium, stability is generally more useful than making sudden changes.
Organic matter buildup and excessive bioload
Uneaten food, fish waste, decaying leaves and accumulated detritus release nutrients as they decompose. These resources can support biofilm development and help sustain diatom growth.
Elevated nitrate or phosphate may indicate excessive organic loading, but the presence of diatoms alone does not reveal which parameter is out of balance. Testing the water is preferable to treating it based only on appearance.
Tratamientos
Adjust feeding
Feed only as much as the animals can consume within a few minutes. Remove visible leftovers so that they do not decompose and release nutrients into the water.
Remove waste and maintain the filter
- Remove dead leaves and decomposing material.
- Siphon areas where detritus accumulates.
- Clean or replace mechanical filter media when it has accumulated excessive debris.
- Clean biological media gently only when necessary, preferably using water removed from the aquarium.
Avoid cleaning the entire filter, replacing biological media and deep-cleaning the substrate during the same maintenance session, as this may destabilise the aquarium.
Keep an appropriate bioload
Keep livestock appropriate for the aquarium volume and filtration capacity. Perform regular water changes based on test results and the needs of the system rather than applying the same schedule to every aquarium.
Available silicates
Diatoms require silicon to build their characteristic mineral cell walls. Dissolved silicon may come from the source water, certain substrates, some rocks, sand or filtration and decorative materials.
Using tap water does not automatically mean that there is a problem. This cause is worth investigating mainly when the film returns quickly after removal or continues for several months.
Tratamientos
Reduce a confirmed silicate source
If a high concentration is confirmed and the problem persists, use water with a lower silicate content, properly remineralised reverse-osmosis water, or a suitable mixture for the aquarium’s inhabitants.
Silicate-removing filter media can also be used. Follow the manufacturer’s instructions and monitor the water parameters to avoid sudden changes in water chemistry.
Identify the source of silicates
- Compare the silicate concentration in the aquarium with that of the source water.
- Check the water supplier’s published analysis when one is available.
- Consider whether the problem began after adding new substrate, rocks, sand or filter media.
- Avoid removing several materials at the same time, as this makes the source more difficult to identify.
Sources
- GBIFGBIF Backbone Taxonomy: Bacillariophyta (diatomeas), grupo taxonómico. Consultado 2026-08.