How to cycle a fish tank

Every new aquarium has the same problem: the filter is sterile. No bacteria live in it yet, and bacteria are what convert fish waste into something the tank can tolerate. Adding fish to an uncycled tank means ammonia builds with nothing to process it. Fish get sick, stressed, or die. This is "new tank syndrome" and it accounts for more dead fish than any disease.

Cycling is the process of growing those bacteria before adding fish. It takes 3 to 6 weeks. There's no way to rush it safely, but there are ways to speed it up.

The short version

Dose the empty tank with ammonia. Wait. Test the water daily. Ammonia will spike, then drop as bacteria colonize the filter. Nitrite will spike next, then drop as a second bacterial population establishes. When ammonia and nitrite both read 0 within 24 hours of dosing, the cycle is done and the tank can hold fish.

What's happening biologically

Fish excrete ammonia through their gills. Uneaten food and decaying plant matter also produce it. Ammonia (NH3) and its less toxic form ammonium (NH4+) are in equilibrium in the water; pH and temperature determine the ratio. Above pH 7.0, more of it exists as the toxic NH3 form.

Bacteria in the genus Nitrosomonas (and related genera like Nitrosospira) colonize filter media surfaces and oxidize ammonia into nitrite (NO2-). Nitrite is also toxic to fish, sometimes more so than ammonia. A second group of bacteria, predominantly Nitrospira in freshwater systems, oxidize nitrite into nitrate (NO3-). Nitrate is relatively harmless at the concentrations found in aquariums and gets removed through water changes or absorbed by plants.

The traditional hobby literature says Nitrobacter handles the nitrite step. Molecular studies from the late 1990s showed Nitrospira is actually the dominant genus in aquarium biofilters. The chemistry is the same either way.

What you need

A test kit that measures ammonia, nitrite, nitrate, and pH. The API Freshwater Master Test Kit (liquid reagents, not paper strips) is the standard. Strips are unreliable for this purpose; the ammonia and nitrite readings need to be accurate to know what phase you're in.

A source of pure ammonia. Options:

• Ammonium chloride powder or solution. Sold specifically for fishless cycling by brands like Dr. Tim's. This is the easiest to dose accurately because the concentration is known.
• Household ammonia. Must be pure ammonia with no surfactants, fragrances, or colorants. Shake the bottle; if it foams, it has surfactants and will wreck the tank. Clear, non-foaming ammonia from a hardware store works fine.
• Fish food. Drop a pinch in daily and let it decompose. This works but is slow, imprecise, and creates a mess. The ammonia method is better.

A heater set to 26-28°C. The bacteria grow faster in warm water. Below 20°C the cycle slows considerably; below 15°C it may stall entirely.

The filter running 24/7. Bacteria colonize the filter media, not the water. Turning the filter off for more than a few hours kills them.

Step by step

Day 1. Set up the tank with substrate, hardscape, filter, and heater. Fill it, dechlorinate, and let the filter run. Dose ammonia to 2 ppm. Some older guides recommend 4 ppm; 2 is enough and avoids unnecessarily long processing times.

Days 2 through ~10. Test ammonia daily. It will stay elevated. Around day 5-10 (sometimes sooner, sometimes later), ammonia will start dropping as Nitrosomonas populations grow. Once ammonia starts dropping, start testing for nitrite too.

Days 10 through ~21. Ammonia should drop to 0 within a day of dosing. When it does, re-dose to 2 ppm. Nitrite will be spiking now, often reading off the chart on the API test (above 5 ppm, the highest the kit reads). This is normal. The Nitrospira population is building but hasn't caught up yet.

Keep re-dosing ammonia to 2 ppm every time it drops to 0. If you stop feeding ammonia, the Nitrosomonas colony shrinks and you lose ground.

Days 21 through ~35. Nitrite starts dropping. This is the slowest phase and the one where people get impatient. The nitrite-to-nitrate conversion is often slower than the ammonia-to-nitrite step. Keep dosing ammonia, keep testing.

Cycle complete. The cycle is finished when you can dose 2 ppm ammonia and both ammonia and nitrite read 0 within 24 hours. Nitrate will be elevated (30-80 ppm is typical at this point). Do a large water change (50-80%) to bring nitrate down before adding fish.

How to speed it up

Seeded media. Borrowing a used filter sponge, a bag of ceramic rings, or even a handful of gravel from an established tank is the single most effective shortcut. The bacteria are already there; they just need to multiply in the new tank. This can cut the cycle from 4-6 weeks to 1-2 weeks.

Bottled bacteria. Products like Fritz TurboStart 700 and Dr. Tim's One and Only contain live nitrifying bacteria. Results vary. Some batches work and cut the cycle to a week; some batches are dead on arrival and do nothing. Keep them refrigerated and check the expiration date. Don't rely on them as the only strategy.

Plants. Live plants, especially fast growers like hornwort, anacharis, and water sprite, absorb ammonia directly from the water column. A heavily planted tank can handle a light fish load before the bacterial cycle is fully established. This is sometimes called a "silent cycle." The plants buffer the ammonia while bacteria slowly establish. It works, but only with genuinely heavy planting and a conservative initial fish load.

Warm water. The 26-28°C range is optimal. Going above 30°C doesn't help and may start harming some bacterial strains.

Common mistakes

Adding fish too early. The #1 mistake. "The water looks clear" is not a sign that the cycle is done. Test the water. If ammonia or nitrite are above 0, the tank is not ready.

Stopping ammonia dosing during the nitrite phase. The nitrite phase is frustrating because the numbers stay high for days. Some people stop adding ammonia, thinking it will help. It doesn't; it starves the ammonia-eating bacteria, and then you have to rebuild both populations when you restart.

Cleaning the filter during cycling. The bacteria live on the filter media surfaces. Rinsing the media under tap water (which contains chloramine in most municipal supplies) kills them. If the filter gets visibly clogged during cycling, rinse it gently in a bucket of tank water.

Using "ammonia-locking" water conditioners as a substitute for cycling. Products like Seachem Prime detoxify ammonia temporarily but don't remove it. The ammonia is still there for the bacteria to eat, which is fine and even helpful. But it doesn't replace cycling; the bacteria still need weeks to establish.

Panicking at the nitrite spike. Nitrite readings above 5 ppm (off the chart on most test kits) are normal during cycling. The Nitrospira population takes time. Patience is the only fix.

After the cycle is done

Do a big water change to bring nitrate down below 20 ppm. Add fish gradually, not the entire stocking list at once. The bacterial colony scaled to process 2 ppm of ammonia; a full tank of fish may produce more than that immediately. Add a few fish, wait a week for the bacteria to adjust, add a few more. The stocking calculator can help plan the order.

The bacteria colony is self-sustaining as long as the filter keeps running and there are fish producing waste. If the tank sits empty for more than a few weeks without an ammonia source, the colony shrinks and you may need to mini-cycle when you restock.