Why does my low-light planted tank suffer from algae?

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Algae causes problems for many fishkeepers. George Farmer looks at a few of the causes.

Algae in planted tanks are caused by numerous factors, either individually, or more likely a combination of factors. You have low-medium lighting so we can rule out excess light.  I suggest an eight-hour photoperiod, as six hours may be too low.

I suspect the main culprit is nutrient control. Too much or too little can result in algae.  You have CO2, which is usually a good thing, but if it’s too low or fluctuating it can trigger algae.

Aim for a stable 30ppm CO2 by using a CO2 drop checker filled with a 4 KH solution and bromo blue pH reagent.  For more information perform an Internet search for “4dKH drop checker”.

Try having your CO2 solenoid switching on an hour before lighting so the plants can immediately access the CO2 as lighting starts.  You can have it turn off one hour before lights off too.

Algae problems are often caused by nutrient deficiencies so consider adding a comprehensive fertiliser. Test for nitrate and phosphate. If these are very low (<5ppm for NO3, <0.5ppm for PO4), then adding a fertiliser with nitrogen and phosphorous can be a good idea. I’ve had success with Tropica Plant Nutrition+. 

Dosing 5ml per day is a good starting point, once you have sorted out your CO2.

You also mention built-in filtration. Does this circulate the water effectively? If not then consider adding more circulation via another filter or powerhead.

Plants do very well with levels of higher circulation, as this distributes the CO2 and other nutrients more effectively.

Algae are also common in aquariums with low plant biomass, so if you only have a few ferns and swords then consider adding more plants. Faster growers are ideal, such as Hygrophila.

This article was first published in the November 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.