Disturbing algae in the marine aquarium and relevant countermeasures – Part 1

Unlike it happens in the freshwater aquariology, in the marine one the algae should not necessarily be considered disturbing, indeed, sometimes even desired. Just think about the beautiful and biologically important calcareous algae, which in many aquaria form on the decorative rocks. However, some species of algae can develop an excessive and hardly controllable growth and therefore represent one of the worst annoyances in the marine aquariology. That can be claimed in the case of the filamentous green algae (Derbesia spp., Bryopsis spp.) and the cyanobacteria, better known as “patinous” algae, but occasionally even for the spherical, (Valonia spp., Ventricaria spp.) and the siliceous ones, as well as for some flagellates known as “golden algae”. The filamentous algae are able to significantly obscure the joy for a marine aquarium. Some thick green coverings may propagate among the corals, growing inside the colonies and opening gashes in the previous dense “polyp meadows” of hard and stoloniferous corals. In some aquaria, filamentous algae rapidly colonize some tiny uncovered areas of the calcareous skeleton of large-polyp hard corals, reducing step by step the tissue of the coral polyps. The Blue-Green algae are also able to strain a marine aquarist’s “resistance”. Some reddish, sometimes even grey or blackish strata extend on the sea bottom and grow forming dense “carpets”, which may also rapidly transfer to the rocky decoration and the trunks of the soft corals, without stopping even in front of the hard corals. What the aquarist aims at, between desperation and evening fits of anger, punctually reoccurs the following morning, since these cyanobacteria have an enormous potential for development. These real plagues do not only represent an aesthetic problem, since the cyanobacteria and the algae influence, in the case of a massive infestation, the aquarium conditions to their advantage and generally in a very serious way at the expense of the higher living beings. So what could be done? The mechanical removal is part of the most important countermeasures, but alone is not normally enough to solve the problem. A change in the conditions of the aquarium results as decisive; without these changes the just reduced algae will rapidly re-grow, reaching the original density.

Filamentous algae

An invasive growth of filamentous algae can be attributed to an “eutrophication” (enrichment of nutritional substances) of the aquarium water. It is first of all nitrates and phosphates. However, the plague of the algae is not exclusively attributable to a problem related to nitrates and phosphates, since the algae need further substances to grow. For example they need iron, carbon dioxide as basic nutriment and of course a sufficient quantity of light energy, possibly of the right spectral composition. In case of a deficit in one of these factors, which can only marginally be compensated by a greater availability of the remaining ones, this will have a limiting effect on the growth of the algae. This fact certainly represents the most important statement in the fight against the infestations of algae: many nitrates and much light alone do not cause a proliferation of filamentous algae, as long as they are contemporary accompanied by an absence of iron and CO2. However, for example, if with a change of water we provide the necessary iron, or we dose more CO2, then an even explosive growth of the algae may take place. Nevertheless, it would be wrong to state that the iron or the fertilization of CO2 cause such problem. The contemporary effect of all the factors and the environment in its global context, cause the infection! For this reason we must control the entire system.

The following factors may facilitate the growth of the filamentous algae:

– high content of inorganic nutrient macro-substances (phosphates, nitrates)

– excessive administration of iron and other trace elements

– much dissolved carbon dioxide (e.g. with a not very efficient calcium reactor)

– long periods of illumination, intense illumination

– light colors favorable for the filamentous algae (inadequate or worn lamps)

– absence of algae-eating organisms

– harmful substances that damage the symbiotic algae o the invertebrates more than the filamentous ones (e.g. too high content of heavy metals, toxic chemical substances)

– much free surface or too few zooxantellated invertebrates (competitors of the free algae!)

The filamentous alga Bryopsis pulmosa, with its rapid growth, is able to suffocate mainly the branch-shaped hard corals.

The filamentous alga Bryopsis pulmosa, with its rapid growth, is able to suffocate mainly the branch-shaped hard corals.

What to do in case of infestation of filamentous algae?

Reduce the organic load of the water

Decreasing the quantity of food sounds rather simple. If however the aquarium is overcrowded, making the fishes starve is relatively useless, without taking account of the ethical implications of such an initiative. So, do not decrease the quantity of food (as long as it has not been excessively administered!), but rather the quantity of animals to feed, since it brings much more advantages! The frozen foods, after having been defrosted, should be briefly rinsed under tap water, because the defrosting liquid contains a high quantity of phosphates! Moreover, high-quality dry vitaminized feeds represent a great enrichment of the diet and at the same time release far fewer substances to the water during the administration compared to the frozen ones.

Cure of the water

With the use of a skimmer ensuring a good yield, and a modest amount of ozone (in a 500-liter) aquarium around 10 millimeters per hour, directly in the skimmer), keep the water clean. The collateral effect of the intense skimming consists in the expulsion of the exceeding carbon dioxide. Check the bottom material and verify the possible presence of an excessive accumulation of sediments. In case, take it from the aquarium (always only step by step!) to gently rinse it in marine water. Test regularly the concentration of phosphates and nitrates. The best values are under 20 mg for nitrates and 0,1 mg for phosphates, per liter.

Use marine water resistant materials

The tubes for aquarium in PVC contain softening substances, which are slowly dissolved polluting the water. Analogue processes of release of toxic substances are also observable in other synthetic materials. Therefore, use only silicone tubes, rigid PVC tubes (for potable water), and synthetic materials that are guaranteed for an employment in marine water. Pay attention even to the backgrounds for artificial aquaria: always ask the manufacturer for a guarantee of a use in a marine aquarium.

Optimizing the lighting

Check the light color and intensity of your lighting plant. Avoid lamps with a too high emission in the band of red and yellow! Use as a test other colors of the light or reduce its intensity, for example by decreasing the switch-on time up to the limit tolerated by the invertebrates. Do not forget to replace the lamps after a period of 8 to 16 months. The spectrum of the HQI lamps moves with the use towards red-yellow.

Nutritional competition through the creeping algae

A good way to contrast the filamentous algae may consist in allowing the proliferation of those with a rapid growth belonging to the genus Caulerpa. Through their strong development, they enter into a sort of food competition against the filamentous algae, determining a reduction of the latter. During this stage any administration of trace elements, especially iron, should be avoided. Neutralise the free CO2, and make the phosphates precipitate. If your aquarium contains too much free CO2, then you should try to increase the pH value by means of the calcium hydroxide. In this way, it bids to the free carbon dioxide making this important factor minimal for the filamentous algae, thus limiting their growth. In addition, it is necessary to consider the effect of the precipitation of the phosphates through the dosage of the calcareous water, so that the filamentous algae have to spend more time and energy to absorb this important nutrient substance. This filamentous alga that spreads in the marine aquarium, is almost not touched and is so robust that it can only be reduced with the use of scissors.

Mechanical contrast

Once the life of the algae has been complicated with the above mentioned methods, it is possible to move on to a mechanical action of contrast. The most consistent “bearings” are eradicated, while the most reduced ones brushed. The removed filamentous algae must be eliminated from the aquarium (aspirating tube, fine net).


The alga-eaters accept much better the last fine remains on the rocks than the original dense bearings. Introduce alga-eating fishes (surgeon fish, rabbit fish), snails (e.g. Nerita and the species Turbo), small hermit crabs and alga-eating sea urchins (e.g. Tripneustes gratilla).