The bubble tube guard is a clear cap which sits over the top of the bubble tube to prevent objects and fish from getting into the bubble tube, located in the center of your biOrb aquarium. Holes in the cap allow bubbles to come out. It is very rare that a fish would go into the bubble tube; however some fish like to try and squeeze themselves into the most unlikely places. Should one of your fish enjoy this behavior we have the bubble tube guard to keep them safe.