Understanding green water

It is so frustrating – you have set up your aquarium, added your beautiful fish then the water turns green. Not just a little bit green, but a bright yet cloudy, pea soup green and you can’t even see your fish. This is the result of what is known as an ‘algal bloom’.

This green water in your aquarium is ugly, and it’s bad for your fish.

It’s simple enough to deal with, but can return in a flash if you don’t do something about the underlying cause.

Aquarium treatments, such as Green Away, are a helpful short-term solution, but it’s important to be clued up on the causes so you can accurately treat these problems and prevent further outbreaks.

There are two primary causes of green water; too much light and too much waste. Algae use light, and chemicals from decaying waste to reproduce and grow. Control these factors and you’ll solve your green water problems. 

Lighting can be a simple fix.

Be certain your aquarium is not exposed to too much light; never place it in direct sunlight and find a way to control the lights fitted in the tank. This will give your aquarium the right amount of light for good fish health and healthy plant growth.

Installing an auto timer or generic household timer lets you control light levels with minimal hassle. A 10-hours on, 14-hours off cycle mimics day and night and is recommended by most fish care experts. 

Controlling the build-up of waste is mostly a matter of keeping up with regular maintenance.

Complete a 25% water change every 28 days as a standard and make sure you remove leftover food and decaying plant matter promptly (as well as any dead fish).

It’s also important to regularly test your water for ammonia, nitrite, nitrate and pH levels. Algae can be a sign of high nitrate; something that can be harmful to your fish but is used by plants & algae as food. As changes in water chemistry are largely invisible, it’s important to run regular water tests to avoid any nasty surprises.

Here are some other things to consider when experiencing green water:


Extreme cases of green water, and the use of some green water treatments, can lead to a lack of oxygen in your aquarium; with deadly results if left unchecked.

Introducing some form of aeration via an air pump and air stone or bubble wand will make your fish much happier and healthier.

Filter maintenance

If green water is bothering your tank, it’s essential keep on top of your filter maintenance.

The filtration system in your tank will help keep the nitrogen cycle going as algae begins to decay. It will also catch the large algal particles which are grouped together by water-clearing treatments. 


Planting can be a good way of preventing further green water problems; they act as competition by using the same food sources as algae.

Looking after your aquatic plants and feeding them with Plant Food will remove the elements algae love, and prevent large outbreaks in the future.

Ongoing prevention

Continue with your regular maintenance routine and consider using a product such as Keep it Clean tablets which reduces maintenance and prevents algae build-up.  

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