Bullfrog spa filters: short thread, long thread, and coreless cartridges

Last reviewed October 27, 2026

Bullfrog’s older threaded cartridges are not one part. Unicel marks 8CH-950 discontinued and says to use 8CH-951^[5]. Pleatco lists PBF35 against a short-thread OEM (10-2785, 2003–2012) and PBF36 against the long-thread OEM 10-2870 (2013–2017)^[2]. From 2017, Pleatco’s spec table adds coreless cartridges PBF17 (OEM 10-00281) and PBF40 (OEM 10-00282), and Leisure Time says those need the correct filter cage and cap installed^[43][42].

Two threaded 8" cartridges, and a discontinued number

Bullfrog used a top-load cartridge with a handle and an SAE thread before it moved some models to a coreless design. The threaded pair is where people order the wrong length.

Read the disagreements instead of averaging them. Unicel’s OEM for 8CH-950 is 10-1035. Pleatco’s spec row for the cartridge it cross-references, PBF35, prints OEM 10-2785, not 10-1035^[2]. Unicel says 8CH-950 is discontinued in favor of 8CH-951 (other retired numbers are in discontinued spa filter numbers). The length on our rows is 6" for 8CH-950 and 5-3/4" for 8CH-951, and Unicel also says 8CH-951 replaces a Bullfrog element with a 6" overall length and a shorter threaded end^[5]. Area is not settled in a single number either: Unicel’s note says the original cartridge is 35 sq ft and that the spec table rates 8CH-951 at 40 sq ft. The compiled size field on our 8CH-951 page shows 50 sq ft. Do not buy this cartridge by the square-foot figure. Buy it by OEM 10-2870 if that is the number on a 2013–2017 long-thread filter, and compare the thread length and the 5-3/4" body with the old one.

There is also a smaller threaded Bullfrog row, 6CH-940, with no OEM number in the catalogs we used: 6" × 8-1/4", semi-circular handle, injection-molded 1-1/2" SAE thread, Pleatco PWW50P3, Filbur FC-0359, 50 sq ft on the row. No model years are attached. If your cartridge is about 6" across rather than 8", start there and measure.

Coreless cartridges: the fabric is not the whole filter

Pleatco’s 2023 spec table lists two Bullfrog coreless cartridges for 2017 onward^[2]. Neither has a Unicel number in the 2026 guide, which is why those pages are filed under the Pleatco part.

• PBF17, OEM 10-00281. Pleatco prints 6-5/8" × 6-1/4", 3-3/8" open top, 4-1/2" open bottom, and calls it the Bullfrog 18 coreless cartridge, 2017 to current. Our row shows 18 sq ft from that Pleatco description. Leisure Time rates the same OEM number at about 27 sq ft, lists Bullfrog STIL spas, and says 2015–2017 STIL spas need Filter Core Cage and Cap Assembly 10-00417. The listing also says the cartridge is coreless and that the correct filter cage and cap must be installed^[43].
• PBF40, OEM 10-00282. Pleatco prints 8" × 6-1/4" with the same style of open ends and calls it the Bullfrog 30 coreless cartridge, 2017 to current (30 sq ft on our Pleatco-based row). Leisure Time rates OEM 10-00282 at 50 sq ft, says 2013–2017 A/R/X spas need the Filter Core Cage and Cap Assembly, and says 2023-and-newer A Series models use Simplicity flat filters instead^[42]. The fitment we publish from that listing is A Series and R Series for 2018–2022, and X Series for 2018–2026.

The area gap (18 vs about 27, and 30 vs 50) is the same kind of catalog disagreement described in why same-size filters show different square feet. For a coreless Bullfrog filter it has an extra consequence: the pleated sock is only half the assembly. Leisure Time’s sentence is to install the correct cage and cap^[43][42]. If the cage is broken, a new sock does not replace the cage. If the spa is a 2023-or-newer A Series, the same listing says the filter is a Simplicity flat filter, not PBF40. Confirm that on the listing or in the spa before you order a coreless cartridge.

Open-end Bullfrog rows that are not the threaded 8"

Bullfrog also appears on two open-end cartridges that other brands use: C-4335 (4-15/16" × 9-1/4", 2-1/8" open both ends, 35 sq ft, Pleatco PRB35-IN) and C-4950 (4-15/16" × 13-5/16", same open ends, 50 sq ft, Pleatco PRB50-IN). Those rows do not carry a Bullfrog OEM number in our table. They are size matches from the catalogs, not a model-year list. If your Bullfrog cartridge is a short 8" threaded filter or a coreless sock, these 4-15/16" parts are the wrong family. The way to tell is the end cap, which is the subject of open-end vs threaded hot tub filters.

What to photograph before you order

1. The OEM number, especially 10-1035, 10-2785, 10-2870, 10-00281 or 10-00282.
2. The overall length. A 6" threaded cartridge and a 5-3/4" threaded cartridge are both “about six inches” if you only glance at them.
3. The thread: short versus long is the distinction Pleatco prints for the 2003–2012 and 2013–2017 Bullfrog 35 cartridges^[2].
4. Whether there is a plastic cage around the media. Coreless cartridges are open fabric over a cage. Threaded cartridges have a handle and a screw bottom.
5. The series badge (STIL, A, R, X) and the year, then check Bullfrog filters by series and year.

Unicel’s maintenance page still applies after you have the right part: cartridges are replaced when cleaning stops restoring flow, not on a calendar alone, and acid washing is the step Unicel warns against doing casually^[4][6]. Installation steps that every brand shares, power off, don’t run the spa empty of a filter, are in how to install a hot tub filter.

Questions

Sources

2. Pleatco 2023 Catalog (Pentair)
4. Unicel: Maintenance & FAQ
5. Unicel 2026 Replacement Guide
6. Unicel: When to Replace a Cartridge Filter (PDF)
42. Leisure Time Inc.: Bullfrog A/R/X series 10-00282 filter cartridges
43. Leisure Time Inc.: Bullfrog STIL series 10-00281 filter cartridges

This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.