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The Top Fin RF-S filter cartridge is a common replacement media used in hang-on-back (HOB) aquarium filters, but many aquarists struggle to find replacements due to limited availability or high cost.
This help guide contains detailed explanations, practical tips, and descriptions of some product options that can be used instead of, or in conjunction with, standard fish tank filter cartridges.
There are a number of substitutes available as replacement media that you can install inside existing Top Fin® fish tank filters and alternative filter systems.
Furthermore, you may prefer to upgrade to another product altogether, such as if you want to move away from proprietary cartridges.
Here's the thing:
All Top Fin® RF-S cartridges are designed to provide mechanical and chemical filtration.
They help to remove physical debris and dissolved organics, and they can also remove odors and other unwanted particles. Still, there are some drawbacks, including:
As a consequence of this, many beginners with a fish keeping hobby look for custom media alternatives, generic options, or entirely different filter types.
Do you want to keep your current filter unit, but replace the media inside the cartridge chamber? If so, here are a few proven options.
Reusable foam sponges provide both mechanical and biological filtration. Plus, you can cut them to fit inside an existing cartridge housing. They trap debris and they provide surface area that encourages beneficial bacteria to grow.
The Aquarium Solutions Bacto Surge Foam Filter Small is one suggested product for hobbyists who want to reduce waste and ongoing cartridge cost. It is a quality aquarium filter foam that functions as a mechanical/bio media.
Pro Tip: You can rinse these types of sponges in old tank water during routine maintenance and then reuse them so you don't need to keep buying disposable cartridges.
You can buy filter media pads as rolls or sheets and then cut them into the required shape to fit proprietary filter chambers. These pads act in the same way as cartridge mechanical media and they can be paired with biological media.
Aquarium Filter Media Pad (Cut To Fit Roll) is a versatile media that can be trimmed to fit inside existing filter housings. They are great for internal chamber setups and for making budget filtration upgrades.
Instead of modifying the cartridge system, you might also consider transitioning to a different filter type altogether. The benefits can include more customisable filtration media, with long-term availability:
These alternatives do not require Top Fin RF-S cartridges and typically allow some customisation of media, especially sponges, bio rings, carbon, and zeolite.
Many hobbyists take standard foam filter pads or sponge and cut them to the shape of the cartridge slot. They also add ceramic bio-rings or bio balls to maximize biological filtration.
Attaching a prefilter sponge on the intake tube helps trap large debris before it reaches internal media, reducing clogging and extending media life.
Some experienced keepers avoid disposable cartridges entirely, preferring reusable foam and dedicated biological media, which can reduce cost and maintain beneficial bacterial colonies more effectively.
Small aquariums (between 5 and 20-gallon fish tanks) may do well with a high-quality hang-on filter and reusable sponge. Larger tanks (30 gallons and above) often benefit from external canister filters with customizable media baskets.
If you want low ongoing cost and reusable media, go with sponge and bio media solutions. For less frequent cleaning and robust mechanical/chemical filtration, a quality HOB or canister filter system is recommended.
Look for filters that offer more than single-stage filtration: mechanical (foam/floss), chemical (carbon/zeolite), and biological media (ceramic rings/bio balls). Good alternatives balance all three functions.
Even if Top Fin RF-S cartridges are unavailable or costly, there are many viable alternatives for maintaining aquarium water quality, such as:
Transitioning your filtration approach can provide better long-term performance, lower ongoing cost, and greater flexibility for media selection on your fishkeeping journey.