Fishless cycling
Fishless cycling exists so that nothing has to live through the ammonia and nitrite peaks. It takes three to eight weeks unaided, and considerably less when seeded with mature media from an established tank.
In one sentence
Establishing the filter's bacterial population by dosing pure ammonia to a target concentration and waiting until the tank converts it to nitrate within 24 hours, before any fish are added.
The method
- Set up the tank fully — substrate, hardscape, filter, heater — and treat the water for chlorine.
- Dose an ammonia source to roughly 2–3 mg/L. Use pure ammonium chloride or a household ammonia solution with no surfactants, perfume or additives.
- Test ammonia and nitrite every day or two. Ammonia falls first; nitrite then rises and eventually falls too.
- Re-dose ammonia to the target each time it reaches zero, so the population is not starved while nitrite is still being processed.
- The tank is cycled when a dose of 2 mg/L ammonia is converted fully to nitrate within 24 hours, with zero nitrite.
- Do a large water change to bring nitrate down, then stock gradually.
Speeding it up
Seeding with mature media — a squeezed sponge, a handful of established substrate, a used filter cartridge — is by far the most effective accelerator, and it is free if you know somebody with an established tank. Keeping the temperature around 26–28 °C and the pH above about 7 also helps, because nitrification slows sharply in cool or acidic water.
Why the cycle appears to stall
- pH has fallen below about 6, at which point nitrification largely stops. Check KH.
- Ammonia was dosed far too high — above roughly 5 mg/L it starts to inhibit the organisms it is meant to feed.
- The ammonia source contained surfactants or perfume.
- The test kit is old or the nitrate reagent was not shaken properly.