Why is my fish tank green?

Fish tanks turn green with algae, which needs light, water and nutrients to grow. Algae spores are airborne, so even if you start off with brand new everything, coldwater, tropical or marine, your tank will go green.

Algae are tiny plant-like dots that can stick to surfaces like the tank glass or even be free-floating in the water. A little algae is natural in any tank – some fish, snails and shrimp are even adapted to eat it, but too much algae is often a sign that there is an excess of light or an imbalance in the tank.

It can make the tank look horrible too and persistent algae problems can even cause some fishkeepers to pack up their nets and leave the hobby altogether. 

Decrease the tank lighting

If you have live plants or corals they need bright light for a fixed photoperiod per day. This should be a minimum of eight hours and a maximum of ten hours or 12 hours including a slow gradual fade in or dawn and dusk. Any more than that and you will get excess algae growth.

If you don’t have any live plants or corals turn the light off when you aren’t sat watching the fish and you will get much less algae growth. The fish won’t mind the ambient room light one bit.

Ensure that no sunlight hits the tank at any part during the day – especially in the summer months, and if its a small tank don’t place it near a window or in a conservatory.

Lower nutrients for non-planted tanks and reef tanks

Keep nutrient levels low by doing lots of partial water changes to lower nitrate levels and add phosphate removing media to the filter. 

Increase fertilisation for planted tanks

Ensure the plants have enough nutrients to be strong and healthy enough to fight algae naturally, so this time actually add nitrates and phosphates with a good brand of complete plant fertiliser. Change the water regularly, brush off and suck out as much algae as you can, and ensure there is good filter circulation and consistent levels of CO2. 

Plecostomus
Bristlenose Catfish are great algae eaters for tropical tanks

Introduce algae eaters to graze algae

Algae eating creatures can help in every sort of aquarium. Add algae eating fish, shrimp and snails to help you keep on top of algae growth. They will work tirelessly, 24 hours per day to help keep your tank looking clean and tidy. 

Use algae removal tools

There are lots of handy tools available to help physically remove algae from your fish tank. Use a combination of algae pads, algae magnets, brushes and blade scrapers. An algae magnet can be used to wipe the front glass daily while keeping your hands dry. 

The three golden rules to lessen green algae

  • Turn the light down
  • Control nutrients
  • Add algae eaters

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Jeremy Gay is an author, lifelong fishkeeper, and aquatic specialist. He's a former editor of Practical Fishkeeping Magazine, UK editor at Reefbuilders, a former aquatic store manager, and has collected fish in Sri Lanka and the Amazon. He's been on tv and radio, contributed to Koi Carp and Gardeners World magazines, been a product tester, a judge, and a product developer. Jeremy is here to guide and advise you on all things tropical, pond and marine, from set-up to stocking, health, feeding to breeding, as well as solving many common fishkeeping problems along the way.


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