What Causes Brown Algae in Fish Tanks?

Algae spores are present in all aquariums. Fish waste provides nutrients to feed the spores, allowing them to multiply and form a visible brown or green slimy layer inside the tank, on decorations and plant leaves. If an aquarium has excess light, too many fish or leftover food, algae can flourish and eventually take over the tank.

  1. Diatom Organisms

    • Insufficient tank lighting causes brown algae, and too much light encourages the growth of green algae. Small patches of brown furry blooms on the glass in a freshwater aquarium are diatom organisms that thrive in low light conditions and excess nutrients in the water. Initially provide 10 to 12 hours of light a day, and increase or decrease lighting levels if brown or green algal blooms appear.

    New Tank Set-Up

    • A new tank can take up to four months for the water chemistry to stabilize. During the first few weeks of a new setup, brown algae is often a problem because fish waste and debris builds up in the water, causing ammonia, nitrate and nitrite levels to rise. Excessive silicates in the water can also encourage brown algae growth in a new tank. This type of brown algae should disappear without intervention when the tank stabilizes at around three to four months.

    Water Quality

    • If brown algae appears in an established tank, poor water quality could be the cause. Ideally, mature water should be free from ammonia, nitrate or nitrite traces and phosphate levels low. Check your water with an aquarium test kit, and if any of these chemicals are present in dangerous quantities, carry out partial water changes once a week until you see an improvement.

    Additional Treatments

    • Use a chemical solution or add algae-eating fish to keep brown algae under control. Brown algae grows in low light, so keep your tank lights on for an extra hour each day and wipe away the blooms until the diatoms stop reappearing. Take care not to provide too much light or green algae will develop.