How Long Does it Take for the Biological Filter to Work in a Fish Tank?

The biological filter of an aquarium isn't a physical filter. The bacteria that eat fish waste -- and convert it to less toxic chemicals -- constitute the biological filter of an aquarium. It generally takes two to four weeks to establish such a filter, but this varies widely based on a number of factors.

  1. Traditional Cycling

    • The process of establishing the biological filter of a fish tank is called "cycling" an aquarium. This typically takes about two to four weeks under normal conditions. In this process, you add a few fish, then measure ammonia and nitrite. When the levels each hit zero, you can perform a 25 percent water change, then add a few more fish and repeat until you've fully stocked your tank. Since this process revolves around living things, it doesn't always follow a perfect timetable, and it can be influenced by things like the number of fish, the size of the tank and the water chemistry.

    Fishless Cycling

    • You can also establish a biological filter through a process called "fishless cycling." In this process, you use household ammonia to kick off the cycling process instead of fish. You add unscented, non-foaming ammonia until the ammonia levels in the tank reach 5 ppm. Then you add the same amount of ammonia per day until the ammonia and nitrite levels reach 0 ppm after less than 12 hours. Then you perform a 25 percent water change and add all of your fish at once. This takes the same amount of time as it would to cycle an aquarium the traditional way, but you can add all of your fish as a group.

    Silent Cycling

    • In silent cycling, you use plants in freshwater aquariums -- or live rock in saltwater tanks -- to speed up the cycling process and smooth the ammonia curves. You still follow the same procedure as you would with traditional cycling, but you include either plants or live rock to help absorb ammonia and nitrite. This results in a much faster cycle, typically more towards the end of the two-week end of the range. This process not only goes quicker, but doesn't have the sharp rises in ammonia associated with traditional cycling.

    Speed Tips

    • Whichever method you use to establish your biological filter, you can use several tricks to speed the process. The bacteria that carry out this process thrive in aquariums with an alkaline pH. So as long as your fish can handle it, try keeping the pH above 7. These bacteria also prefer water with a tropical temperature between 78 and 82 degrees Fahrenheit. The best way to speed the process is to add gravel or other decorations from an aquarium that already has its biological filtration established.