Perfect for freshwater aquariums, the Fluval Betta Moss Ball is a 2-in-1 product that both looks beautiful and acts as a chemical filter media, controlling phosphate levels inside your tank. The ball contains adsorption resin that helps to reduce many of the compounds that encourage organic material to accumulate. The moss ball has a naturally soft texture that won't injure delicate betta fins and will effectively treat a tank up to 20 gallons in size. Position the ball in the flow of water for best results.