How Algal Treatments Work

There are several varieties of algae that can plague your aquarium or pond; they range from tiny suspended cells to long filamentous forms or producing a scum on the surface. Their colours range from green through to red, brown and black.

Algal spores are ubiquitous and are just hanging around waiting for the right conditions to proliferate. This occurs when there is sufficient light and nutrients (nitrate, phosphate, iron) for them to establish and replicate.

There are a number of 'tools' available to help combat the problem if your initial attempts failed (as outlined in the fact sheet 'Algal Problems?'). I will briefly describe the science behind how each of these 'tools' work and their application in the pond and or aquarium.

UV clarifiers

This is a very high tech and expensive piece of equipment, but it is the safest and most effective method to clear suspended algae and other minute organisms from the water. The sleeve contains a special bulb that produces ultraviolet radiation that damages the DNA and RNA of cells passing through, killing them in the process. The dead algal cells then clump together and either decompose on the bottom or get sucked out by your filter. The bulbs wear out and require replacing at least annually.

Flocculants

These are chemicals (usually aluminium sulphate) that cause the suspended algal cells to clump together, which then either sink to the ground or float to the surface. This can then be manually removed with a fine net or sucked up by the filter. It is important to remove the algae after treating because any decaying material will provide nutrients for the remaining algae and create another algal bloom.

These chemicals may alter certain water parameters like pH (if the KH is too low). This is only a temporary measure to remove algae. This should be used in conjunction with lowering nutrient levels by reducing fertilising, feeding and performing more regular partial water changes.

*To minimise problems due to reduction in dissolved oxygen, increase aeration the day before you begin treatment and continue to do so throughout the treatment.

Ion-exchange Resins

High nitrates and phosphates can predispose your pond to algal blooms. Ion-exchange resins remove nitrates and swaps them for other ions. Phosphate removers are either aluminium (white) or iron oxide based (rusty black). It is recommended that ion-exchange resins only be used if your water source contains high levels of phosphate and nitrate.

Barley Straw

It is believed that the aerobic decomposition of barley straw releases chemicals (humic acids that are later converted to hydrogen peroxide in the presence of sunlight) that inhibit algal growth and that the decomposition encourages the growth of microbes that feed on algae (you must ensure adequate water flow over the material as anaerobic decomposition of straw produces harmful breakdown products).

This treatment option takes a while to work and is dependent on the water temperature and the amount of sunlight. For best results, the straw should be relatively loose and placed in a warm, sunny position (shallow margins of the pond). These chemicals are produced after soaking for one month and continues for six months. Literature suggests 5g of barley straw per 1,000L with a minimum of 100g being used. Some companies have gone a step further and bottled the stuff - Barley Straw Extract. This natural treatment is safe for plants, pets and children.

* If the straw turns black in colour and produces a rotten egg smell (hydrogen sulphide), it is indicative of anaerobic decomposition and it is no good. It is important to increase aeration to enhance the breakdown of the straw and for the well-being of the fish (decomposing matter removes oxygen from the water).

Algicides

Copper and terbutryn-based compounds are, in effect, aquatic herbicides. They are very effective and produce quick results, however, there are many disadvantages. Copper causes irritation to fish causing excess mucus production, making it difficult for fish to breathe. Terbutryn-based products are not environmentally friendly. Some algicides can stunt the growth of aquatic plants or even kill them. For this reason, algicides are generally recommended only for unplanted ponds.

*The dead algae still needs to be removed manually or by mechanical filtration.

Biological Control

This method relies on the addition of microbes to control algae. Most of these work by boosting the populations of certain bacteria which remove the nitrate and phosphate the algae use as their food source. This method takes a while to be effective. In heated tanks, fish such as bristlenose catfish and the chinese algae eaters are fantastic at removing attached algae. Another method of biological control is by planting aquatic plants to 'out-compete' the algae for light and nutrients (see 'Shading').

Shading

Algae, like most plants, require light to grow. Providing partial to full shading can reduce algal growth significantly. This shading can be achieved by placing slats across the surface of the pond, building a roof, heavy planting around the pond or planting water lilies in the pond. Be careful not to smother the surface excessively because reducing the air/water interface can diminish oxygen/carbon dioxide exchange and greatly reduce wind aeration.

Coloured Pigments

Adding colour to the water reduces the amount of light and/or the right light wavelengths available to the algae (similar effect to shading). However, it will also have the same effect for your submerged aquatic plants and make it difficult for you to see your fish. Also, it may stain certain types of cement, rock and plastic. Seaworld at Gold Coast uses this method for their dolphin pools.

Chlorine Solutions

This should only ever be used for water features that do not contain fish or plants. Chlorinated water is usually safe for your four-legged pets to drink from, but is lethal to aquatic organisms including fish. (Tap water is chlorinated and this is the reason why you use conditioners before adding to the aquarium.)

*Chemicals should never be the sole treatment. It should be used only after, or in conjunction with, management strategies.

By Dr Richmond Loh - Last updated 16 November 2012