Green Algae is an umbrella term for the many types of algae which are common in all types of freshwater aquariums. Green algaes often boom and then decline and are a normal part of every tank, however while present, green algaes can be troublesome for the planted tank and can easily cover the glass or cloud the water.

Quite often green algae is indicative of two major components being out of balance: Lights and nutrients. For example, some types such as “green water”, can imply an imbalance such as too much sunlight or, more importantly, excessive nutrients. Others, such as “green spot algae” or “hair algae” are hard to avoid even in healthy aquaria, but are especially common in new aquariums.



Green Water:

Green water is made up of algae particles are suspended in the water (they don’t require anything to attach to like most types of algae) and is often found in high-bioload aquariums like Goldfish aquariums, or tanks exposed to a lot of sunlight. The fix for Green water is usually a simple one, and involves changing some basic conditions in the fish tank:

- Water changes! We’re going to sound like a broken record with this recommendation, but removing excess nutrients is really important for fixing many sorts of algaes. If water changes have no effect, it could imply one (or several) of three things: 1. Your water source contains high Phosphates or Nitrates, 2. The organisms in your tank are making a large amount of waste very quickly or 3. Your lighting is significantly out of balance. There are often multiple components adding to algae growth, however even if water changes appear to make no difference, they should help reduce your nutrient levels and your fish will always appreciate the clean water!

- If your tank is near a window, try blacking out the back and sides of the tank with black card, plastic or paint. If this is not possible, removing the lights on top of your tank or only using them for viewing may be of help. Cutting back your light hours to be between 5-10 hours a day can be helpful in preventing several kinds of algae.

- If your tank is outside and reducing the excess light isn’t possible, a UV sterilizer can work wonders for preventing Green water.

- While fixing the problem, a 3-4 day blackout can be very effective for significantly reducing the amount of Green water present. A blackout will help to prevent too much algae from “bouncing back” when you make the changes above. However, it’s always worth remembering that a large die-off of algae can create a Nitrate spike and a water change is recommended afterwards. In tanks with inadequate filtration, an Ammonia or Nitrite spike is also possible.



Green Spot Algae:

Green Spot (GSA) commonly grows on plants, ornaments and glass and is typically very difficult to remove without the aid of a razor (an old credit card is also often used instead). These algae types often “boom” in new tanks, but are also often caused by intense lighting or long lighting hours. Many also feel that it is caused by an imbalance of nutrients, including too low CO2 and too low Phosphates. While these algaes are normal and a small amount is nothing to be worried about, the following methods can be used to help control it:

- Water changes or or methods or reducing the build-up of nutrients such as Nitrates.

- It may be recommended to increase phosphates to maintain a level around 2-3ppm.

- Increasing CO2 using either a liquid carbon supplement, DIY CO2 or pressurized CO2 can be used, as this will also help aid in the growth of your plants. Growing plants are often very desirable for decreasing algae levels.

- Reducing lighting hours can be very beneficial, and many tanks only need lighting to be on for anywhere between 5-10 hours. Some also feel that raising artificial lighting upwards can help, as keeping the light close to the tank can cause intense lighting on the glass. Using good output angles, such as with the aid of reflectors on Fluorescents, can also help intensify the lighting in the correct direction and away from the glass.

- Fast growing plants are less likely to develop this algae on their leaves, and thus they can be used to help with the uptake of nutrients without being out-competed by the algae.

- Bristlenoses can be used to help eat this algae, but it is always important to remember that the addition of fish adds to the bioload of your tank (and thus the overall nutrient levels). Every fish also has specific needs and should not be added to a tank that they are not suited to live in.


Michelle Wimp 1

Above: Green Dust Algae and Brown Diatoms growing on a rock. Photo Credit: Michelle Whimp

Green Dust Algae:

Green Dust Algae develops a “dusty” film over ornaments, plants and glass, and is also very common in a new tank. Poor lighting quality, long light hours and too intense lighting may be to blame, however when present in a new tank, it is often recommended to simply leave the algae as it tends to go away after a couple of months as the tank matures.



DSC 5027

Above: Green Hair Algae

Green Hair Algae:

“Hair Algae” can refer to many types of algae, but here we are referring to the “fuzzy” algae that grows short and attaches itself to ornaments - quite often driftwood. Like Green Spot algae, Hair algae can be due to intense lighting or long lighting periods and reducing light intensity is often recommended. High nutrient levels are also believed to be a cause, along with low CO2. Hair algae is typically present in low amounts, however it can quickly take over ornaments. Some, though, even like to use this algae to enhance “aquascapes”!

- Reduction of light intensity, such as by raising lights or changing lighting type or quantity (eg. going from a “high light” T5HO to a “low light” T8) may help.

- Like GSA, lighting hours can play a big part. Reducing the time your lights are on even by an hour may provide results. 5-7 hours is typically recommended for a high-light tank, 7-10 hours is typically recommended for low-light tanks.

- Water changes, again! Water changes will help reduce your nutrient levels, and thus gives the algae less nutrients to feed on.

- Increasing CO2 whether by using a liquid supplement, DIY system or pressurized system, can help increase the nutrient uptake by plants and thus may help plants out-compete this algae.

- Siamese Algae Eaters are known to eat this algae, however these fish get large and need groups of 5+. Adding these fish will also add to the bioload of the tank, which may increase the levels of different kinds of algae not eaten by the SAEs.



Green String Algae:

Green String Algae can refer to several kinds of algae which grow long filamentous strands which are not attached to any surfaces. It can vary in texture, from a wool-like texture to slimy mats. String algae can become very problematic as it blocks out light to plants and picks up debris, however it can also be beneficial as a host to microbial colonies and as a food source for some fish. Despite there being several different kinds of algae under this description, the treatment to reduce these algaes is often similar.

- Like other algaes, intense lighting or long lighting hours can impact the growth of this algae.
- Increasing flow will help to prevent this algae from forming mats in the tank, and thus prevents it from blocking out light to other plants.
- Excess nutrients can play a big role for this algae to thrive. The “slimy” type of Green String algae, specifically, thrives on excess Iron. It may be worth reducing any fertilizers dosed, or switching to substrate fertilizers such as “root tabs”.
- Many recommend manually removing this algae with a tool such as a toothbrush, which will easily pick up the strands of algae.
- Some fish, such as American Flag Fish, thrive on this algae as a part of their diet. However, like all other fish, these fish have specific requirements including requiring groups of 5+ and preferring harder waters.


Blue-Green Algae:

Blue-Green Algae is in fact not even an algae at all, and is a type of bacteria called Cyanobacteria, which you can read all about here.


Still got Algae problems? Feel free to contact us or perhaps use the forum hosted by the FNZAS (Federation of New Zealand Aquatic Societies) for opinions from other local hobbyists.