How to Spot Filter Problems in a Fish Tank

Written by bonnie conrad
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How to Spot Filter Problems in a Fish Tank
Keep your filter clean and change the cartridge regularly. (Aquarium image by crossgolfing from

The filter is a critical part of your aquarium. If the filter is not functioning correctly, the quality of the water in the tank will suffer, and the health of your fish could suffer as a result. Watching for the telltale signs of filter problems can preserve the quality of your water and make your fish happier and healthier.

Skill level:


  1. 1

    Remove the filter media from any power filters that hang outside the tank. Remove the top cover from the filter and carefully lift the filter cartridge out. If the filter appears blocked or dirty it should be replaced. The filter might also develop algae growth that requires it to be replaced.

  2. 2

    Check the condition of the filter floss inside any corner filters. The filter floss is white when it goes into the filter, so if it appears dark or discoloured it should be replaced.

  3. 3

    Watch the condition of the water closely. If you notice visible debris floating in the tank, that is an indication that the filter either is not working, or that the size is insufficient to keep your tank clean.

  4. 4

    Test the water using a comprehensive water-testing kit that looks for ammonia, nitrates and other common fish tank contaminants. High levels of any contaminant mean that the filtration system is not working properly.

  5. 5

    Watch the water flow from your power filters. These filters work by drawing water out of the tank into a small box that hangs on the outside of your tank. The water is then run through the filter cartridge and returned to the tank via a small waterfall that runs into the aquarium. If a power outage stops the filter, it might not recover when the power goes back on. If the filter is not running, fill the back of the filter with water until the water begins to flow back into the tank.

Tips and warnings

  • Change the filter cartridge once a month.
  • Keep a supply of filter cartridges in your aquarium stand.

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