This filter is designed to fit on top of the basket under the floating weir that is in many of the spas built today. Keep in mind that this filter can plug very quickly and stop the flow of water through the main filter. If the filter is left in the spa, no filtration will be happening, and the water could go bad very quickly. WARNING: Do not use the Filter Saver Top if your spa uses a circulation pump to pull the water through the filter. These pumps are not strong enough to pull through the Filter Saver Top after it starts to get dirty and will not give the results you want.