How to cycle a fish tank

Fishless cycling from start to finish, plus fish-in cycling when you have no choice. Ammonia targets, phase timelines, how to speed it up, and troubleshooting a stalled cycle.

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

Add ammonia to the empty tank. 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 the last ammonia addition, 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. Add ammonia to bring it to 2 parts per million. Some older guides recommend 4 parts per million; 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 each ammonia addition. When it does, top it back up to 2 parts per million. Nitrite will be spiking now, often reading off the chart on the API test (above 5 parts per million, the highest the kit reads). This is normal. The Nitrospira population is building but hasn't caught up yet.

Keep topping ammonia back up to 2 parts per million 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 add 2 parts per million of ammonia and both ammonia and nitrite read 0 within 24 hours. Nitrate will be elevated (30-80 parts per million 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 truly 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 additions 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 parts per million (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 parts per million. Add fish gradually, not the entire stocking list at once. The bacterial colony scaled to process 2 parts per million 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.

Fish-in cycling (when you already have fish)

Sometimes you end up with fish before the tank is cycled. A gift, a rescue, an impulse buy. The fish are in the tank, ammonia is rising, and fishless cycling isn't an option anymore. Fish-in cycling is possible, but it requires daily attention and water changes to keep ammonia and nitrite low enough that the fish survive while the bacteria establish.

Daily water changes. Test ammonia and nitrite every morning. If either reads above 0.5 parts per million, do a 25-50% water change with dechlorinated water to dilute it. In the early stages, you may need to change water daily or even twice daily. This is tedious, but it keeps the fish alive.

Reduce feeding. Feed every other day instead of daily, and feed small portions. Less food means less ammonia production, which gives the bacteria time to catch up.

Add a detoxifying conditioner. Seachem Prime or Fritz Complete detoxifies ammonia and nitrite for 24-48 hours without taking them from the water (the bacteria can still use them as food). This buys time between water changes. Add it after each water change.

Add bottled bacteria. This is one situation where bottled bacteria products are worth trying. Fritz TurboStart 700 in particular has a strong track record for jump-starting fish-in cycles. Add it directly to the filter intake so it reaches the media.

Don't add more fish. The biofilter is already overwhelmed. Adding more animals makes it worse. Stock the rest of your fish list after the cycle completes (both ammonia and nitrite consistently at zero between water changes).

Fish-in cycling takes the same 3-6 weeks as fishless cycling. The difference is that you're doing daily water changes and conditioner additions throughout, which is substantially more work. The fish may show stress signs (clamped fins, reduced appetite, sluggishness, susceptibility to disease) even with diligent water management. Some fish survive fish-in cycling without visible harm. Others carry permanent gill damage from ammonia exposure that shortens their lifespan.

Fishless cycling is always the better option when you have the choice.

When the cycle stalls

Sometimes the cycle appears to stop progressing. Ammonia drops normally, but nitrite stays high for weeks with no sign of declining. Or ammonia never starts dropping at all despite daily dosing.

Nitrite stuck high for more than 3 weeks: The Nitrospira population is growing slowly. Common causes: temperature too low (below 22 C slows them significantly), pH too low (below 6.0, nitrifying bacteria slow dramatically; this can happen if KH is depleted by the acid produced during cycling), or chloramine in the water supply (even trace amounts from water changes can suppress bacterial growth).

Check and fix: raise temperature to 26-28 C, test KH and add buffer if below 2 dKH, and double-check that you're dechlorinating all water added to the tank. Some people find that a small water change (20%) with well-dechlorinated water breaks the stall by reducing accumulated nitrite (which at very high concentrations can inhibit the bacteria trying to process it).

Ammonia not dropping after 2 weeks: The Nitrosomonas population isn't establishing. Check that the filter is running and water is flowing through the media. Check temperature (needs to be above 20 C). Verify that the ammonia source is actually adding ammonia (test immediately after dosing to confirm the reading changes). Make sure you haven't accidentally used ammonia with surfactants, which coat filter media surfaces and prevent bacterial colonization.

If everything checks out and the cycle still hasn't started after 3 weeks, try adding seeded media from an established tank or a fresh bottle of live bacteria. Sometimes the ambient bacterial population in a new tank just takes longer to establish than average.

Maintaining the cycle long-term

The bacterial colony in your filter adapts to the ammonia load it processes. If you stock 10 fish and the biofilter matures to handle that load, then you add 10 more fish at once, you'll see a mini-cycle: ammonia and possibly nitrite spike for a few days while the bacterial population expands to handle the increased waste. This is why the standard advice is to add fish gradually (no more than 2-3 at a time with a week between additions).

Events that can disrupt an established cycle:

Medication. Antibiotics (erythromycin, kanamycin) kill nitrifying bacteria along with the target pathogens. If you need to medicate the main tank, expect ammonia to rise during and after treatment. Monitor daily and do water changes as needed. Better option: treat in a separate hospital tank to avoid disrupting the main tank's biofilter.

Power outages. The bacteria in your filter need oxygenated water flowing over them. Without power, the filter becomes stagnant and oxygen-depleted within 4-8 hours. The bacterial colony starts dying. After a prolonged outage (12+ hours), regard the system as partially cycled: test ammonia and nitrite daily for 1-2 weeks and reduce feeding until the colony recovers.

Over-cleaning the filter. Never rinse filter media under tap water (chloramine kills bacteria). Never replace all the media at once. Rinse gently in a bucket of old tank water during water changes. If replacing sponge or ceramic media, do half at a time and wait 3-4 weeks before replacing the other half.

Extended absence without feeding. If you go on a 2-week vacation and the fish aren't fed (or are fed minimally by an auto-feeder), ammonia production drops. The bacterial colony shrinks to match the reduced load. When you return and resume normal feeding, a small ammonia spike may occur as the bacteria ramp back up. Feed lightly for the first 2-3 days and monitor.

The stocking calculator accounts for bioload when recommending fish additions, helping you avoid overwhelming an established cycle.