35 balls Used to grow bacteria in the filter system of marine tank the bacteria eat the bad things in the water and help maintain your levels nitrites etc. if you wish you may pickup from sw16 london streatham. we are 1 min walk from streatham Common train station. thank you for looking. Please don`t be put off because they are not new. soon as you put them in the tank they get dirty then cover themselves with bacteria.
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