The filters in spas and hot tubs are made up of several layers. The topmost layer is usually a brown, paper-bag-like material that takes in water and debris. Below that is a heavier material that holds the dirt and debris. Over time, these filters will become dirty and lose their filtering ability. In addition, if the filter is transferred to a new owner, there may be additional build-up of dirt and debris on the filter. The pleated part of the filter may also become flat, losing its fibrous sheen. If any of these signs are present, it's advisable to have your filter replaced.