Looks like it's a WaterWay Clearwater II FC125 filter -- check their catalog, and verify that, before you purchase the cartridge below:
http://www.waterwayplastics.com/wp-c...0.0510_web.pdf
The Unicel cartridge guide IDs the correct replacement as a either Unicel C8412 or an C8413 (pg 59). You'll have to measure the cartridge: the C8412 is 23 5/16 inches; the C8413 is 28 3/16 inches. The Filbur FC-1293 (23 5/16") or FC-2575 (28 3/16") are options as well.

23 5/16" long
Unicel C-8412
FIlbur FC-1293

28 3/16" long
Unicel C-8413
Filbur FC-2575

Be very careful if you purchase without using the links below; Asian made cartridges are sold deceptively online, at Amazon and elsewhere, with descriptions like "replaces Unicel C8412"! If you want further info, here are the links to the Unicel and Filbur cartridge guides:
http://www.filburmfg.com/uploads/res...%20Catalog.pdf
http://www.unicelfilters.com/pdf/replacement_guide.pdf