Cycling Fish: What They Do and Which Species Work Best
By Kenji Tanaka — Freshwater systems and koi husbandry
Before the fishless cycle became the standard recommendation in the early 2000s, "cycling with fish" was the method. You set up a tank, you stocked a few tough little fish, you fed them lightly, and you waited six to eight weeks while nature built your biological filter. It still works. It is also the slowest, riskiest, and most ethically fraught way to start a tank. This article explains what cycling fish actually do in the nitrogen cycle, which species are commonly used (and why), and a step-by-step protocol if you choose to do it anyway.
What "cycling fish" actually means
A "cycling fish" is a fish intentionally added to an uncycled aquarium so its waste feeds the bacteria that will eventually process that waste. The fish is, in effect, a living ammonia source. In a fully cycled tank, two bacterial colonies handle fish excreta:
- Nitrosomonas spp. oxidize toxic ammonia (NH₃/NH₄⁺) into nitrite (NO₂⁻).
- Nitrobacter and Nitrospira spp. oxidize nitrite into the much less toxic nitrate (NO₃⁻).
Neither colony exists in meaningful numbers on day one. They have to be cultivated, and the only practical source of ammonia in a new tank is either a fish, a bottle of ammonium chloride, or decaying organic matter (an "organic" fishless cycle using shrimp or fish food).
The chemistry, written out:
$$\text{NH}_3 \rightleftharpoons \text{NH}_4^+ \quad (\text{pH- and temperature-dependent})$$
$$2\,\text{NH}_3 + 3\,\text{O}_2 \xrightarrow{\text{Nitrosomonas}} 2\,\text{NO}_2^- + 2\,\text{H}^+ + 2\,\text{H}_2\text{O}$$
$$2\,\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{Nitrobacter/Nitrospira}} 2\,\text{NO}_3^-$$
Every fish you add produces NH₃ through gill diffusion, urine, and feces. The total ammonia nitrogen (TAN) output of a small fish at 25 °C is roughly 0.05–0.10 mg NH₃-N per gram of body weight per day. That is enough to start a cycle, but it is also enough to burn gills, damage fins, and kill the fish if the bacterial colonies are not yet established.
The role of the cycling fish in a fish-in cycle
A cycling fish is not a "tool" in any humane sense — it is a living animal that experiences real physiological stress from ammonia and nitrite. Its job in the cycle is to:
- Produce a steady, low-level ammonia source so *Nitrosomonas* can colonize filter media.
- Survive long enough for nitrifying bacteria to reach a population large enough to handle the bioload.
- Generate nitrate that you can later remove via water changes.
The risk is that the ammonia and nitrite peaks between days 10 and 30 routinely hit 1–4 ppm TAN and 2–8 ppm NO₂⁻. Both are acutely toxic: 0.25 ppm NH₃ (the un-ionized fraction at pH 8.0, 28 °C) is enough to cause gill necrosis in many species within 96 hours. This is why species selection matters — you want a fish that tolerates poor water better than the species you ultimately plan to keep.
Which species work, and why
The classic cycling species share three traits: small adult size, low metabolic waste per gram of body mass, and a documented tolerance for sub-optimal water. The four I would consider for a 75–150 L (20–40 gal) tank:
| Species | Adult size | Why it works | Why it fails |
|---|---|---|---|
| Zebra danio (Danio rerio) | 4–5 cm | Extremely active, fast metabolism recovers quickly from short ammonia spikes; tolerant of 18–28 °C | Small and skittish; needs a lid |
| White cloud mountain minnow (Tanichthys albonubes) | 3–4 cm | Prefers cooler water (18–22 °C), which keeps un-ionized NH₃ low | Slow to establish dominance; very small |
| Cherry barb (Puntius titteya) | 4–5 cm | Calm, hardy, handles low O₂ better than most barbs | Slightly more sensitive than danios |
| Tiger barb (Puntigrus tetrazona) | 5–7 cm | Robust waste producer (good for faster cycle), tolerates wider pH | Semi-aggressive; not for community tanks |
Avoid for cycling: any catfish (Corydoras, plecos — they need established biofilm), livebearers (guppies, mollies breed during the cycle, fry die), bettas (too sensitive to NH₃), and any fish you actually want to keep. Goldfish and koi produce so much waste that the ammonia curve goes off the chart in week 2.
The traditional ratio is 1 small cycling fish per 38 L (10 US gal). I would go lower — 1 per 75 L (20 gal) — and feed only 2–3 times per week with a tiny pinch of food. Less food = less ammonia = lower peak toxicity = more humane cycle.
The dosing math and the equipment that actually helps
If you want the predictability of a fishless cycle but the moral compromise of fish-in, you can hybridize: a single fish + controlled bottled ammonia (Seachem Ammonia Chloride, API Ammonium Chloride). Target 1.0–2.0 ppm TAN during week 1, then let the fish's own waste carry the cycle.
You will test every 48 hours for the first three weeks. I recommend the API Freshwater Master Test Kit (≈700 tests per box, ~$30) for ammonia, nitrite, nitrate, and high-range pH. The Seachem Ammonia Alert badge is a useful continuous indicator but is not a replacement for liquid tests.
Filter media choice matters more than filter brand. You want maximum surface area: coarse sponge (Aquaclear sponges, Eheim Substrat Pro, or generic bonded filter floss) in a high-flow chamber. Avoid carbon-only filtration during cycling — carbon strips ammonia-binding organics and slows colonization.
Water-change math: if TAN exceeds 0.5 ppm or NO₂⁻ exceeds 1.0 ppm, do a 25–30% water change with dechlorinated, temperature-matched water (Prime by Seachem at 2 drops per 38 L neutralizes 1 ppm ammonia and 1 ppm nitrite for 24–48 hours — useful as a stopgap between water changes).
A 6-week fish-in protocol (if you do it anyway)
- Week 0: Tank, filter, dechlorinator, heater set to 24 °C, lights on 6 h/day. Add 2 zebra danios per 75 L.
- Weeks 1–2: Test every 48 h. Feed 1 small pinch every 2 days. If TAN > 1 ppm, do a 25% water change.
- Weeks 3–4: Expect NO₂⁻ to rise sharply around day 18–25. If NO₂⁻ > 2 ppm, add Seachem Prime and do a 30% change. Do not add more fish.
- Weeks 5–6: NO₃⁻ should appear and climb. When TAN and NO₂⁻ both hold at 0 ppm for 7 consecutive days with normal feeding, the cycle is complete.
- End of week 6: Rehome the cycling fish (return, donate, or move to a true quarantine). Add your intended stock gradually — 3–4 small fish per week.
Modern alternatives worth knowing
If the ethics or the math of fish-in cycling bother you, the fishless cycle is now the textbook method. Dosing 2 ppm pure ammonium chloride (DrTim's Aquatics NH₃Cl or similar) every other day for 4–5 weeks builds a stronger, more tolerant bacterial colony because bacteria grown under stable, non-toxic conditions multiply faster than bacteria grown under fluctuating, toxic conditions. You can also seed the new tank with established media from a friend's filter (1 L of mature sponge can cut the cycle to 7–10 days) or use a commercial bacterial starter (API Quick Start, Seachem Stability, Fritz Zyme 7) — though starter products shorten, they rarely eliminate, the cycle.
A hybrid I now prefer: fishless cycle with seeded media, then a 14-day quarantine for the first livestock. The cycling fish is no longer necessary in 2026, and most of the old species lists exist only because we did not have reliable liquid ammonia in the hobby a generation ago. If you already have the fish, use them. If you are starting from scratch, dose a bottle instead.
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Article word count: ~1,490. Reviewed against current hobbyist practice; not a substitute for a 50 L hospital-tank setup if you have sick or quarantined animals.
