Many older Filters were manufactured with one of the above thread-in adapters (which means that they screw in to your hot tub) and these adapters were detachable from the body of the filter.
The filter has an open (usually slotted) bottom hole, and the adapter filter would fit snugly into that bottom hole. This was a bad design because the filters would detach from the adapter, and would end up floating on top of the water, and the manufacturer stopped making these filters years ago.
If you filter falls into this category you can replace it with a filter that has screw-in threads on the bottom, but first you would need to check to see if you have an old adapter screwed into your hot tub, and then gently try to remove it.
If this applies to you, you may want to try to check to see if you have an adapter installed in your hot tub by gently trying to unscrew the adapter - you may want to use a washcloth to get a better grip
It's very important to remember to NOT force anything, because it may be a permanent part of your hot tub, but if it does unscrew, you can then measure the adapter and find the correct filter that screws into your Hot Tub by using our Filter Finder by Size