Chemical filter media: what’s worth using, and what’s a waste

A filter does three separate jobs, and it helps to keep them straight. Mechanical media (sponge, floss) traps particles. Biological media (ceramic, sponge, sintered glass) houses the bacteria that process ammonia and nitrite. Chemical media — carbon, Purigen, phosphate and ammonia removers — adsorbs or binds dissolved substances out of the water. The first two are essential. The third is optional, situational, and the one the industry most wants to sell you.

The honest headline: in a healthy, established planted tank, most chemical media is unnecessary. Good stocking, feeding, planting and water changes do the real work. Chemical media is a tool for specific jobs — and a couple of types can do more harm than good. So rather than “which should I run,” the better question is “do I have a job that needs one?”

Placement and flow

Wherever you put chemical media, two rules apply. First, it goes after the mechanical stage, so the water reaching it is already stripped of gunk — otherwise it clogs and stops working. Second, it needs flow through it, not around it: adsorption depends on water actually contacting the media, so a bag wedged in a dead corner does very little. In a canister, that usually means a media bag sitting after the sponge/floss and before or alongside the biological media, in the main flow path. It does not replace biological media — it sits with it.

Activated carbon

The classic. Activated carbon works by physical adsorption — its vast internal surface area traps dissolved organic molecules, tannins, some medications, chlorine and odour compounds. That makes it genuinely useful for a few jobs: removing medication after a treatment course, stripping tannins you don’t want (from driftwood, say), and clearing odours or the haze of a brand-new setup.

But two things temper it. First, it exhausts within a few weeks — once its sites are full it stops adsorbing and simply becomes (mediocre) biological media, so running the same carbon indefinitely does nothing chemical. Second, it isn’t selective: the dissolved organics it removes include the humic substances and tannins that, as the case for tinted water explains, can actually benefit fish. Running carbon permanently to keep water sparkling therefore has a real cost. Carbon is a tool for a job, not a default — the same conclusion the over-clean trap reaches about chasing sterility.

Purigen

The reader favourite, and the most genuinely useful of the bunch for a planted tank. Purigen is a synthetic macroporous adsorbent resin that pulls soluble organic nitrogenous waste out of the water — ideally before it breaks down into ammonia and feeds the nitrogen cycle. It polishes water to a striking clarity and helps hold down the organic load between water changes. A handy feature: it starts off-white and darkens to brown as it exhausts, so it tells you when it’s spent.

The stand-out advantage is that, unlike carbon, Purigen is regenerable. When it browns, a soak in a bleach solution (roughly one part household, unscented bleach to one part water) for around 24 hours restores it, after which it can be reused many times.

Even so, Purigen is a polish and organics-control tool, not a substitute for water changes — and, like carbon, it removes tannins and dissolved organics, so blackwater keepers should skip it. Whether “crystal-clear” water is a health goal at all is worth questioning: see does crystal-clear water mean healthy fish.

Phosphate removers (GFO and alumina)

Here is the one to be most wary of in a planted tank. Phosphate-removing media — granular ferric oxide (GFO) or aluminium-oxide products — bind phosphate out of the water. In a reef tank fighting nuisance algae on near-zero nutrients, that can make sense. In a planted tank it is usually the wrong move. Aquarium plants need phosphate (a common target in fertilised tanks is roughly 0.5–2 ppm, though many low-tech tanks run leaner), and stripping it out starves them. Worse, the folk belief that low phosphate “starves algae” is largely a myth — phosphate limitation tends to weaken your plants first, which then lose the competition and let algae in. If you are dosing fertiliser into one end of the tank and running a phosphate remover at the other, the two are fighting each other. For almost every planted tank, phosphate removers are unnecessary at best and counter-productive at worst.

Ammonia removers (zeolite / clinoptilolite)

Zeolite — usually the mineral clinoptilolite — grabs ammonium (the dissolved NH₄⁺ ion) by ion exchange, a capacity that is well documented (Ghasemi et al., 2018). That makes it a legitimate emergency and transport tool: a hospital tank with no established filter, moving fish, or buying time during an ammonia spike while a filter matures. Two caveats, though: it works best in soft, fresh water, and loses much of its capacity in hard or salt water, where competing ions crowd it out — so it is unsuitable for marine tanks and weaker in very hard water. And in a cycled tank it is the wrong habit: it removes the very ammonium your biological filter needs as food, so it can stall or starve the cycle, and once saturated it stops working and can even release ammonium back. It regenerates — a soak in a strong salt (brine) solution recharges it — but for a running planted tank, a mature biofilter does this job better and for free.

Softening resins and peat

A niche corner. Ion-exchange and deionising resins soften or purify water for soft-water species, while peat does the opposite of carbon — it adds tannins and gently softens the water and lowers pH for blackwater setups. Both are specialist choices rather than general-purpose media, and most keepers will never need them.

When chemical media is actually worth it

Chemical media is the seasoning, not the meal. The mechanical and biological stages — covered in the filter media guide — plus sensible maintenance are what keep a tank healthy. If you had to run a filter with only two of the three media types, you’d drop the chemical stage every time.

Keeping it working

• Activated carbon — replace every 3–4 weeks if you’re using it for a purpose; home “regeneration” (baking it) doesn’t meaningfully restore aquarium carbon, so don’t bother. Once it’s spent, either swap it or pull it out.
• Purigen — regenerate when it turns brown: 24-hour bleach soak, rinse, heavy dechlorinator soak, rinse again until there’s no bleach smell. Reusable many times; discard once it stops whitening up.
• Zeolite — regenerate in a strong salt solution, or replace. Don’t leave saturated zeolite in a tank.
• Phosphate remover — replaced when spent, but in a planted tank the better move is usually not to run it at all.

1. Ghasemi, Z., Sourinejad, I., Kazemian, H. & Rohani, S. (2018). Application of zeolites in aquaculture industry: a review. Reviews in Aquaculture, 10(1), 75–95.
2. Seachem Laboratories. Purigen product technical information (manufacturer data — mechanism and regeneration procedure).