Why do the Fish in a New Aquarium Keep Dying?

Setting up a new aquarium can be an exciting experience---that is, until all the fish in the tank begin slowly dying off. This phenomenon, often called "New Tank Syndrome," is extremely common in new aquariums but can be entirely prevented if the tank is set up and stocked more carefully.

  1. New Tank Syndrome

    • A healthy, established aquarium is home to much more than just fish. Fish produce ammonia in their waste, which is processed by bacteria and converted into nitrite. Nitrite and ammonia are both very harmful to fish in even low quantities, but healthy tanks build up enough beneficial bacteria colonies to break the nitrite down even further, into nitrate, which is much less harmful. A new tank lacks these vital bacteria, so ammonia just builds and builds, eventually poisoning fish to death.

    Starting a Cycle

    • The only way to prevent fish from death by ammonia poisoning is to cycle the tank. After setup, a new tank should either be stocked with a few small, hardy fish to begin the process, or treated with a commercial product designed to cycle the tank. Adding just a few fish acts to supply ammonia for the bacteria colonies in very small quantities relative to the body of water, and choosing hardy fish helps prevent unnecessary deaths.

    Ensuring the Cycle is Finished

    • Cycling can take up to a month or as little as two weeks, but during this time, tank parameters should be monitored every day using an aquarium test kit. Whether the cycle involves fish or a commercial product, cycling is completed when the test kit shows no more ammonia, and a nitrate reading registers. This means that enough bacteria are present to convert ammonia into nitrate. At this point, a 30 percent water change is recommended, and the tank is ready to stock.

    Stocking and Care

    • Stocking should always be done slowly--never more than a fish or two per week. This will allow the bacteria colonies present in the tank to build and adjust to handle the increasing amount of ammonia introduced to the tank. The tank should be cleaned regularly, with 10 percent to 20 percent water changes weekly to ensure the removal of nitrates. Never rinse the filter media with anything other than used tank water, as this could kill your bacteria colony and start the cycle over.

    Other Causes

    • Rarely, other culprits are behind the deaths of fish in new aquariums. If your tank has been cycled properly and new fish introduced slowly, a parasite or other disease may be the culprit. Common illnesses include ich (a parasite that leaves white spots on fish), velvet (gold patches of scales concentrated on the head) and fungi, and can all be treated with a combination of commercially available medication and good water quality. Make sure you purchase fish from reliable sellers to reduce the possibility of introducing illness.