Antimicrobial (blue) hot tub filters: what the AM, RA and -M part numbers mean
An antimicrobial hot tub filter is a normal pleated cartridge whose media (or end caps) carries a treatment meant to stop bacteria and mold growing on the filter itself. The suffix tells you which maker: Unicel uses AM (formerly RA)[5], Filbur uses M on its blue cartridges[31], and Pleatco’s -M (Microban) numbers are retired, with the 2023 catalog pointing each one to the standard number without the -M[2]. The size, fittings and square feet match the standard cartridge. Under EPA’s treated-article rule the treatment may only protect the filter, not the people in the tub, so sanitizer and cleaning schedules do not change[59][61].
Blue spa filters confuse people for two reasons. The part number on the end cap has an extra letter that a search will not always match, and the word “antimicrobial” sounds like it does the sanitizer’s job. This page covers both. The numbers come from the Unicel 2026 Replacement Guide[5], the Pleatco 2023 catalog[2] and the Filbur catalog[31].
The suffixes, maker by maker
| Maker | Suffix | Status in the source we read | Example |
|---|---|---|---|
| Unicel | AM | Current. Spec notes say “Manufactured with antimicrobial filter media.”[5] | 8CH-202AM, “formerly 8CH-202RA” |
| Unicel (older) | RA | Earlier suffix. Unicel’s notes give the RA number as the former number of an AM part[5]. Filbur’s cross reference still prints RA numbers[31]. | 8CH-852RA → 8CH-852AM (now discontinued, use 8CH-66) |
| Filbur | M | Current. Blue “Premium Antimicrobial Cartridges”, “more than 50 sizes”[31] | FC-2390M (Rainbow 50 in-line) |
| Pleatco | -M | Retired. Listed under “Deactivated Pleatco part numbers” with a standard replacement[2] | PRB75-M → PRB75 |
Hot Tub Outpost’s listing for PRB75-M describes the -M as “the blue Microban color”, says the standard version without Microban is PRB75, and marks the -M as discontinued[58]. That matches the Pleatco list.
What the treatment claims, and what it cannot claim
Filbur says its antimicrobial protection prevents the growth of bacteria on the cartridge, is applied during manufacturing, will not wash out or wear away, and lasts as long as the cartridge. Fewer bacteria on the filter means fewer odors, algae and mold fouling it. Filbur gives the blue cartridges a one-year warranty[31]. Filbur’s standard (white) cartridges also have what it calls reinforced antimicrobial end caps, which it says repel odor-causing bacteria and mold[31]. So a white Filbur cartridge is not “untreated” in every part.
In the US, a product treated with a pesticide to protect the product itself is a “treated article”. It is exempt from pesticide registration only if its claims stay limited to protecting the article[61][60]. EPA’s consumer page gives sample wording for such products: the antimicrobial properties are built in to inhibit bacteria that may affect the product, and they “do not protect users or others against bacteria, viruses, germs, or other disease organisms”[59]. For a hot tub that means:
- A blue cartridge does not replace bromine or chlorine, and it does not change the sanitizer level you test for. See sanitizer types.
- It still loads with oils and scale and needs the same rinse and soak. See how to clean a hot tub filter and how often to clean it.
- None of the three catalogs gives a longer replacement interval for the treated version. The intervals in how often to replace a hot tub filter apply.
Unicel AM cartridges in the 2026 guide
These are the spa-size AM numbers in Unicel’s 2026 price index, with the area and application Unicel prints[5]. Where we have a cross-reference page, the part links to it.
| Unicel AM | Area | Application Unicel lists | Note |
|---|---|---|---|
| C-3310AM | 8 sq ft | Master Spas Eco-Pur inner element | Used with C-7330 or C-8341 outer cartridges. See Master Spas Eco-Pur filters. |
| C-4314AM / C-4327AM | 13.5 / 27.5 sq ft | Saratoga | Need the existing tabbed thread adapter in the skim well (not furnished). Not eligible to ship into Canada. |
| C-4950AM | 50 sq ft | Rainbow in-line | Same size as C-4950 |
| C-6430AM | 30 sq ft | Watkins | The price index says “C-6430 USE C-6430AM” (see below) |
| C-9450AM / C-9489AM | 150 / 200 sq ft | Waterway | OEM 817-1150M / 817-1200M. Waterway’s own numbers also end in M. |
| 4CH-949AM | 50 sq ft | 13-1/2" long, 2" SAE thread | |
| 5CH-352AM / 5CH-502AM | 35 / 50 sq ft | Marquis | Standard 35 sq ft version: 5CH-352 |
| 5CH-902AM | 90 sq ft | Marquis (OEM 20499) | 5-3/16" × 30", bar handle, 2" MPT. Dealer sizes differ slightly (see the Marquis lookup). |
| 6CH-940AM | 40 sq ft | Standard version: 6CH-940 | |
| 6CH-960AM / 6CH-961AM | 52 / 60 sq ft | Jacuzzi Premium | Standard version: 6CH-960 |
| 8CH-202AM | 120 sq ft | Diamante | Formerly 8CH-202RA |
| 8CH-852AM | 85 sq ft | Diamante | Discontinued by Unicel. Unicel says use 8CH-66. |
The C-6430 note (Hot Spring 71825 / 31489)
Unicel’s 2026 price index has a line that reads “C-6430 USE C-6430AM”, while its spec table still prints the standard C-6430 (6" × 10-1/2", 1-15/16" open bottom, 4 oz media, 30 sq ft)[5]. In other words, Unicel’s ordering list sends the plain C-6430 to the antimicrobial version of the same size. If a dealer ships a blue C-6430AM against an order for the Hot Spring 30 sq ft cartridge, that is the reason, and the fit is the same. Our Hot Spring C-6430 page carries the note. Hot Spring spas that use a Tri-X ceramic cartridge in that position need the Tri-X part, not either Unicel number.
Retired Pleatco -M numbers and their replacements
The Pleatco 2023 catalog lists these spa -M numbers as deactivated, each with the replacement Pleatco names[2]. In almost every case the replacement is the same number without -M.
| Deactivated | Pleatco replacement | Prefix family |
|---|---|---|
| PRB25-IN-M, PRB35-IN-M, PRB50-IN-M | PRB25-IN, PRB35-IN, PRB50-IN | Rainbow |
| PRB37-IN-M | PRB50-IN (not a PRB37-IN) | Rainbow |
| PRB75-M, PRB8.5-M, PRB14.5-M | PRB75, PRB8.5, PRB14.5 | Rainbow |
| PLBS50-M, PLBS75-M, PLBS100-M | PLBS50, PLBS75, PLBS100 | Leisure Bay |
| PLB50-M | N/A (no replacement given) | Leisure Bay |
| PMA10-M, PMA25-M, PMA40-F2M-M, PMA45-2004R-M | PMA10, PMA25, PMA40-F2M, PMA45-2004R | Master Spas |
| PJW23-M, PJW60TL-F2S-M | PJW23, PJW60TL-F2S | Jacuzzi / Whirlpool |
| PDO75-2000-M | PDO75-2000 | Dimension One |
| PSD125-2000-M | PSD125-2000 | Sundance |
| PCST80-M, PCST120-M | PCST80, PCST120 | Coast Spas |
| PSG27.5-M | PSG27.5-XP4 | Saratoga |
| PCS50N-M, PCS75N-M | PCS50N, PCS75N | Coleman |
| PBF35-M | PBF35 | Bullfrog |
| PLAS35-M | PLAS35 | |
| PJC147-M | N/A |
Three rows break the drop-the-M rule: PRB37-IN-M goes to a 50 sq ft PRB50-IN, PSG27.5-M goes to PSG27.5-XP4, and PLB50-M and PJC147-M have no replacement. Several standard numbers have pages here: PRB25-IN, PRB35-IN, PRB50-IN, PMA45-2004-R, PJW60TL-F2S, PDO75-2000, PSD125-2000 and PBF35.
Filbur M cartridges: examples and the Unicel numbers Filbur crosses them to
Filbur’s antimicrobial table prints a Unicel cross for many of its M cartridges, usually an RA number[31]. Examples:
| Filbur | Use Filbur lists | Area | Unicel number Filbur prints |
|---|---|---|---|
| FC-2390M | Rainbow in-line 50 | 50 sq ft | C-4950RA |
| FC-2375M | Rainbow Dynamic in-line 25 | 25 sq ft | C-4335RA |
| FC-2810M | Sundance MicroClean 80 | 80 sq ft | C-8380RA |
| FC-3965M | Caldera 100 (Pleatco PCD100W-M) | 100 sq ft | C-8399RA |
| FC-3960M | Watkins 65 (PWK65-M) | 65 sq ft | C-8465RA |
| FC-0536M | Bullfrog 10-1035, 7-7/8" × 5-3/4", 2" SAE | 35 sq ft | |
| FC-1007M | Master Spas Eco-Pur outer, new style (PMA45-2004-R-M) | 40 sq ft | |
| FC-1001M | Master Spas Eco-Pur inner, 4" × 7-1/8" | 10 sq ft | Unicel’s inner, C-3310AM, is rated 8 sq ft |
Where the RA numbers lead. Filbur crosses its cartridges to 29 different Unicel RA numbers. Only six of them have an AM listing in Unicel’s 2026 guide: 5CH-352, 6CH-940, 6CH-960, 8CH-202, C-4950 and C-6430[31][5]. For the other 23, among them C-4335RA, C-8380RA, C-8399RA and C-8465RA, Unicel 2026 lists only the standard number. If you search an RA number and find nothing, drop the RA and look up the plain number (for example C-8380, C-8399, C-8465, C-4335). That gets you the same size in standard media.
Swapping a blue cartridge for a white one (or back)
Every cross in the three catalogs pairs the treated and untreated versions at the same diameter, length, end fittings and area[5][2][31], with the exceptions listed above. Read the number on the end cap, remove the AM, RA, M or -M, and check the result in the part-number search. Then confirm the measurements as described in how to find your spa filter size. If your spa maker’s manual calls for an antimicrobial cartridge by part number, follow the manual. Why two same-size spa filters can still carry different area ratings is covered in different square feet on the same size.
Questions
What does AM mean on a Unicel spa filter?
It is Unicel’s suffix for a cartridge “manufactured with antimicrobial filter media”. Older Unicel numbers used RA for the same thing; the 2026 guide gives 8CH-202RA as the former number of 8CH-202AM.
Are blue Pleatco Microban (-M) filters still made?
The Pleatco 2023 catalog lists the -M numbers as deactivated and gives a replacement for each, usually the same number without -M (PRB75-M to PRB75, PBF35-M to PBF35). PRB37-IN-M goes to PRB50-IN, and PLB50-M has no replacement.
Does an antimicrobial hot tub filter mean I can use less sanitizer?
No. Under EPA’s treated-article rule the treatment may only protect the filter itself. EPA’s sample wording says such properties do not protect users against bacteria, viruses, germs or other disease organisms. Keep the same sanitizer levels and cleaning schedule.
Can I replace a blue antimicrobial cartridge with a standard white one?
The catalogs pair each treated cartridge with an untreated one of the same diameter, length, fittings and area, so the fit is the same. Drop the AM, RA, M or -M and look up the plain number, unless your spa’s manual calls for the antimicrobial part.
Why did I get a C-6430AM when I ordered a C-6430?
Unicel’s 2026 price index says “C-6430 USE C-6430AM”. It is the antimicrobial version of the same 6" × 10-1/2", 30 sq ft Hot Spring / Watkins cartridge.
Sources
- Pleatco 2023 Catalog (Pentair)
- Unicel: Maintenance & FAQ
- Unicel 2026 Replacement Guide
- Filbur Catalog & Reference Guide
- Hot Tub Outpost: Pleatco PRB75-M Microban filter listing (discontinued; standard version PRB75)
- US EPA: Consumer Products Treated with Pesticides (treated articles)
- US EPA: PR Notice 2000-1, Applicability of the Treated Articles Exemption to Antimicrobial Pesticides
- eCFR: 40 CFR 152.25, Exemptions for pesticides of a character not requiring FIFRA regulation (treated articles, paragraph (a))
This guide summarizes published manufacturer and public-health guidance; always follow your own spa’s owner’s manual. See our disclaimer.