Why is my tank turning black?


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Bob Mehen helps a reader understand the reasons his tank is turning black.

Q: I have had a tropical tank for around five years. It’s 40 l in volume, housing only basic stuff like Neon tetras and Amano shrimps or Balloon mollies from time to time. But, since moving to Liverpool, the tank has started turning black and I can’t explain it. I had a white dragon rock and now it’s black. The plants are starting to follow, although they don’t seem to be dying; it’s as though they are just covered with something. I do a 25% water change every week and use API Stress Zyme and Stress Coat when I do. I’ve also got an issue with hair algae now, too. 

Parameters using my API 5-in1 test strips are usually 120°H, 40°KH, pH 7, and nitrite and nitrate 0ppm. Any help pointing me in the right direction would be much appreciated. 


A: Bob advises: From the photo you provided, it looks like your tank is suffering from a combination of blue-green algae (cyanobacteria) and hair algae with perhaps a little black brush algae (BBA) thrown in for good measure. All algae is generally the result of too much of something, such as too much light, or too many nutrients. Stripping or cleaning of the tank, even if only partial, prior to the move, and the potential change of tapwater chemistry may have been enough to trigger this infestation. Test strips are good for a broad idea of what is happening chemically in your aquarium, but liquid test kits tend to give more detailed, reliable results. 

Phosphate is a major contributor to cyanobacteria blooms and is often the result of excess decaying organic material in the aquarium. It may be worth adding a phosphate removing media to your filter to help with this, as well as doing some more intense cleaning, especially under the wood décor you have. A 48-hour black-out of the lights will also help knock the other algae on the head. 

You don’t mention feeding your plants but, counter-intuitively, it is worth adding a commercially prepared aquarium fertiliser to boost plant growth, which will help them to outcompete the algae for light and nutrients. 

Finally, if you haven’t done so already, then setting your tank lights to a defined daytime period of no more than eight hours each day, using a timer, usually helps, as excessive light is a key algae trigger.