Hang-on-back (HOB) overflow boxes can easily become clogged by algae and fail, causing your tank to overflow. Since the tubing on these overflow boxes is clear, algae can actually grow inside them. If your aquarium is equipped with these types of overflows, you can do a few things to prevent their clogging. For starters, regular maintenance will do wonders. You could also use black tubing or put some form of a cover over the box to block out the light. Some overflow boxes utilize lifter pumps to help keep the siphon going.