PWK25 / C-6600 / FC-3930 / Watkins 31115
25 sq ft replacement filter cartridge • 6.25″ OD × 11.875″ long
Compare on Amazon
PWK25 replacements on Amazon
The buttons below pull up Amazon results for each cross-reference part number. Same physical cartridge, different stickers, often different prices. Cross-shop and pick the cheapest option that has solid recent reviews.
Cross-Reference & Interchangeable Part Numbers
| Brand | Part Number | Type |
|---|---|---|
| Pleatco | PWK25 | Primary |
| Unicel | C-6600 | Equivalent |
| Filbur | FC-3930 | Equivalent |
| OEM | Watkins 31115 | Original Equipment |
Always verify dimensions and end-cap type before purchasing a replacement.
Frequently Asked Questions
What are the dimensions of the PWK25?
The PWK25 is 6.25 inches in outer diameter, 11.875 inches long, with an open top and an open bottom. Total filtration area is 25 square feet. Before ordering, pull your existing cartridge and measure it yourself, or check the label inside the housing. OD and length both have to match within about an eighth of an inch, and the end-cap style needs to match exactly so the cartridge seats properly when the lid closes.
What other part numbers are interchangeable with the PWK25?
PWK25 is the same physical cartridge as C-6600, FC-3930, plus OEM versions like Watkins 31115. Same dimensions (6.25" × 11.875"), same open top, same open bottom. The Pleatco, Unicel, and Filbur boxes ship the same media inside, just with different stickers. The only thing to watch is the end-cap style: if a cross-reference has a slightly different cap design, it may not seat right in your housing.
Is the Watkins 31115 the same filter as the PWK25?
Yes. Watkins 31115 is an OEM number, the code the tub or pool builder stamps on the cartridge they ship. The filter itself is made by Pleatco, Unicel, or Filbur, so Watkins 31115 and PWK25, C-6600, FC-3930 all point to one interchangeable cartridge. Buy whichever is cheapest with solid recent reviews, and just check that the dimensions match your old cartridge before you order.
How often should I replace the PWK25?
Most filter cartridges last between 12 and 24 months. Pull the PWK25 every few months for a hose rinse, soak it in cleaner quarterly, and replace it when the pleats are torn, channeling, or won't come clean. Bather load, water chemistry, and how often you actually clean it all matter more than the calendar.
How do I clean the PWK25?
The PWK25 has open ends on both top and bottom, so water flows straight through. Pull the cartridge from the housing and hose it down. Hit the pleats from above at about a 45-degree angle so water rinses debris out instead of packing it deeper, and work top to bottom while rotating the cartridge. Skip the pressure washer. The high pressure tears or collapses pleats, and once that happens you've lost media that doesn't come back. Every few months (or at every water change, whichever comes first), do a deep clean: overnight soak in a hot tub filter cleaner, then a thorough rinse. If your cartridge has a closed end cap, let it dry before reinstalling so you don't trap moisture inside the cartridge core.
Which hot tubs and spas use the PWK25?
Pull your existing cartridge and read the part number off the end cap, or check the label inside the filter compartment. That's the most reliable way to confirm compatibility.
Dimensions
Specifications
| Outer Diameter | 6.25″ |
|---|---|
| Length | 11.875″ |
| Top Type | Open (no end cap) |
| Bottom Type | Open (no end cap) |
| Filtration Area | 25 sq ft |
| Primary Brand | Pleatco |
| Part Number | PWK25 |
Fitment notes
3-inch top and bottom openings. Watkins and Unicel rate this cartridge at 45 sq ft while Pleatco rates the same part at 25 sq ft, so the two figures describe one filter, not two. Replaces Watkins OEM 31115.