Diatoms. What are they and how can I get rid of them?

Posted in All About the Water


Diatoms are photosynthesising algae, they have a siliceous skeleton (frustule) and are found in almost every aquatic environment including fresh and marine waters, soils, in fact almost anywhere moist. They are non-motile, or capable of only limited movement along a substrate by secretion of mucilaginous material along a slit-like groove or channel called a raphe. Being autotrophic they are restricted to the photic zone (water depths down to about 200m depending on clarity). Both benthic and planktic forms exist. Diatoms are formally classified as belonging to the Division Chrysophyta, Class Bacillariophyceae. The Chrysophyta are algae which form endoplasmic cysts, store oils rather than starch, possess a bipartite cell wall and secrete silica at some stage of their life cycle. Diatoms are commonly between 20-200 microns in diameter or length, although sometimes they can be up to 2 millimeters long. The cell may be solitary or colonial (attached by mucous filaments or by bands into long chains). Diatoms may occur in such large numbers and be well preserved enough to form sediments composed almost entirely of diatom frustules (diatomites), these deposits are of economic benefit being used in filters, paints, toothpaste, and many other applications.


Brown algae are not a new occurrence in aquariums. Many hobbyists have, over time, dealt with outbreaks of them and have had to either remove them by siphoning them out, cleaning the tank's panes, or some other method. Diatoms, or brown algae, appear at some point or another in just about every aquarium and are part of the initial set up problems that everyone goes through. They are not limited in appearance though to the first few weeks a new tank is set up.


After the initial stages, when the aquarium is cycled, diatoms and brown algae should not reappear. If they do, there is something wrong with the water chemistry. Note that in this article the words diatoms and brown algae are used interchangeably and refer to all forms of such algae, not just the filamentous or patchy ones but also the encrusting varieties.


It should also be noted that diatoms are “usually” brown in color, but not always. Sometimes green dotted forms appear as well. The reasons for the difference in coloration has to do with the type of pigments that the alga contains (the pigment determines the color).


It has been found that diatoms appear because of the presence in the water of silicates and silicic acid. To avoid having brown algae in the tank, all the hobbyist needs to do therefore is remove the silicate and the silicic acid both from the tank's water, and from the raw water used to prepare water for water changes, top-offs of evaporated water, and so on.


Note, as indicated, that it is not enough to treat only the aquarium. The raw water (e.g. tap, well, deionized, reverse osmosis) needs to be treated as well. If you do not, silicic acid will get into your tank's water and will, eventually, lead to the appearance of brown algae because the levels of silicate and silicic acid will slowly build up.


Diatoms need to be removed from the tank for several reasons, because they:

  • make the aquarium look very unsightly and do so very quickly.
  • grow and proliferate quite rapidly.
  • overgrow anything they can, including rock and sand as well as glass and acrylic.
  • may be hard to remove from acrylic panes without scratching it.
  • lower the oxygen levels when they die off in the aquarium.
  • pollute the water with more silicate which makes more diatoms grow.
  • give rise to anaerobic areas if many of them accumulate in one spot.
  • and so on.

How does Silicate and Silicic Acid get into your Aquarium?

Many water sources contain silicates or silicic acid, or compounds that contain these elements and these compounds eventually break down, adding silicates and silicic acid to your aquarium's water.


In aquariums where sandy or other forms of substrate are used, silicate can leach “out” of these compounds, and enter the tank's water very quickly. It usually only takes a few weeks for this to happen (at the maximum). Often it happens much sooner. The latter is especially the case if silica sand is used as the substrate in the aquarium (this is too often the case,unfortunately). Although less common now than a year or so ago, hobbyists were adding “play sand” to their tanks to build up a substrate. Play sand and so-called Blasting sand are very high in silicate and should be avoided if you wish to keep silicates low.


I have made reference to removing silicates and silicic acid from the water. Please understand this to mean that you should attempt to keep their levels as low as you can. You will never really be able to remove “all” of these compounds. Recommended safe levels are below 0.5 ppm, or half of one part per million.


These silicate compounds then start building up in your tank's water since nothing is removing them, unless you actually take some measures to prevent them from doing so. When they build up to sufficient levels or concentrations, brown algae start to appear in patches first (often on the glass or acrylic panes of the tank). Mind you the patches can be hard, filamentous, slimy and can take on various other forms, depending on which particular diatom is growing in the tank.


Many hobbyists are under the impression that using reverse osmosis water or deionized water will solve the problem, and that doing so will remove the silicates and silicic acid from that water. This is, unfortunately, not the case. Such units, even if combined, will remove silicate and silicic acid for a very short period of time only and, then, the silicate and silicic acid will get through the membrane and the resins and will start to build up in the tank.


By a short period of time I mean in a matter of days, depending on how much silicate and silicic acid the water you are treating actually contains. Tests have shown that, when treating water with 14 ppm of silicate, the silicate and silicic acid came through the membrane and the resins used in combination with the R.O. unit, after less than 100 gallons of water had passed through the filtration set-up (lab. notes ref. 95/6/-012)


This is really not much water when you consider that most hobbyists believe that using reverse osmosis, deionization or a combination of both, gives them water quality of very high purity levels. Obviously this is not so, and that belief is a totally false one”. R.O. and D.I. remove silicates and silicic acid for “some” time” only, after which the membrane of the reverse osmosis filter and the resins in the deionizers let the compounds through.


The conclusion to draw from this is that, regardless of how you filter your water, silicates and silicic acid will find their way into your aquarium's water, and you are going to have to take steps to remove it. This is discussed in the next section for both the water in the tank and the raw water. If you do not, diatoms will appear in your tank.

How do you remove these compounds?

First of all, we need to take into account that two sources of water need to be treated.

  • The water already IN the aquarium
  • The water used to perform water changes, I refer to this as the raw water.

Treating the water already in the tank:

This is very simply done by using a silicate removing compounds and ensuring that all the water in the tank flows through the compound on a continuous basis and that good contact between the water and the compounds exists so they can effectively and efficiently deplete the water passing through them of silicate and silicic acid.


To ensure that this happens, the silicate removing compounds needs to be positioned in such a manner in the general aquarium water flow that all that water will indeed flow through it and not just over and by it. To ensure this, you must carefully look at what the best place will be in your particular case and filtration set up. There may be several possibilities. Pick the one where you estimate that the water will come into maximum contact with the silicate removing compounds as this will ensure better and more efficient removal of the unwanted “nutrients” for diatoms.


If the water does not come into proper contact with the silicate removing compounds, the silicates and silicic acid will not be properly, or entirely, removed from the water. If the silicate and silicic acid are not removed, they will build up and, soon, diatoms will appear in the aquarium.

If this is the case, you have obviously not achieved your goal and you have under utilized the efficiency of the silicate removing compounds that you placed in the aquarium (usually in a bag or fine mesh pouch).


Remember too that these compounds do not last forever and that they need to be changed. At the slightest appearance of brown filaments on your tank's panes you should replace all the compounds. Once you get a feel for how long this takes, you can replace them a few days earlier and prevent the diatoms from appearing altogether.


If you need to clean the tank's panes do so in the following manner:

  • Use a sponge (natural or artificial).
  • Wipe from the bottom to the top trying not to wipe any diatoms in the water.
  • Rinse your sponge real well after you have reached the top of the pane.
  • Go to the next area and again wipe from the bottom to the top.
  • Rinse your sponge again well and preferably several times.
  • Continue as above until your tank is clean.
  • All the panes should be cleaned.


The above cleans the tank's upright areas but does not clean whatever diatoms grow on rocks and in other areas. Some of these can be siphoned out of the tank right away and others can be removed after you use the silicate removing compounds for a while. As you keep treating the tank itself with compounds, the concentration of silicate and silicic acid drops. This causes existing diatoms to die off and allows you to siphon them out.


Pre-Treating Water

The process is simplest when you use two buckets and a kitchen sieve. Place the silicate removing compound in the sieve (on top of white filter floss if necessary) and place the sieve on top of one bucket. The other bucket should be filled with the water to be treated. Now pour water through it until your bucket is full. Move the sieve to the empty bucket and pour the water through again. Do so a total of 5 times.


Your water is now silicate free and is ready to be used.


Change the silicate removing compound after you have poured a total of 50-75 gallons of water through it. The exact amount that can be poured through each batch of compound depends on how much silicate the water you are treating contains. In order to determine that the treated water is indeed low in silicates, you may wish to acquire one of the several tests that are now commercially available. Check magazines and commercial web sites for availability and pricing. A web site that contains a very large number of links is the TAT one. They are in the Links area. Note that the site was recently named: The Best Commercial Aquarium Site for 1996.


Make sure that you use a compound that is specific for silicate removal and that says so on the label. It should not be a multi-purpose product. The best results are obtained with compounds that state that all they remove is silicate and silicic acid and nothing else. That is how you will obtain the maximum silicate and silicic acid removal rate.


Compounds do not last forever. They have a maximum aborption capacity. Depending on how much silicate and silicic acid enters your water, you will have to replace them from time to time. Testing can tell you when this is needed. Alternatively, if you see small brown filaments appear on the tank panes, you know you have waited too long to replace the compound.


Follow the above guidelines for the water already in the tank and the raw water you use and problems with diatoms will either not occur or be very easy to control rapidly. Several commercial products to achieve silicate and silicic acid removal are available. Again, I refer you to magazines, cybermags and to commercial web sites on the Internet.


Keeping the aquarim diatom free is not complex or complicated but requires that you do pay attention to the factors that lead to their appearance outlined in this article. As you have surmized you can easily do so. The key is to do it. Do not wait till your aquarium is overgrown by all forms of diatoms. Intervene before it happens by removing silicates and silicic acid from the tank water and also from the raw water you use.

Note that to minimize the additon of these compounds to the tank you should also make sure that additives you use do not contain them. One source that is frequently overlooked is salt. Using a high quality salt will ensure you are not adding these compounds without being aware of it and suddenly being faced with outbreaks of brown algae. Remember that prevention is easier to implement than remedial actions. It is also far less expensive to prevent them from growing than to remove them once they appear in the tank, especially if they appear in large numbers and in different forms (patchy, filaments, branching filaments, encrusting, slimy and so on).


This information is a collections of various excellent resources. I have included the URL's of the original sources if you'd like to explore them further. I take no credit for the research, all credit should be granted to the original authors. I only wanted to consolidate this information into a single explanative souce for the benefit of our forumites.


- Submitted by dvross