NEW CARDS ADDED DAILY | COLLECT EVERY SPECIES

ALLCORALS CORAL CARD COLLECTION

How to Cycle a Reef Tank: Complete Nitrogen Cycle Guide

Everything you need to know about cycling your reef aquarium — from understanding the nitrogen cycle to choosing the fastest cycling method, navigating the ugly phase, and knowing when it’s safe to add livestock. Compiled from the best practices of Bulk Reef Supply, Dr. Tim’s Aquatics, Fritz Aquatics, Reef Builders, and the reef keeping community.

18 min read Sources: 15 expert articles

1. What is the Nitrogen Cycle?

The nitrogen cycle is the biological process that makes your reef tank safe for life. Without it, toxic waste from fish and coral feeding would accumulate and kill everything in the tank. Understanding this cycle is the single most important piece of knowledge for any new reef keeper [1].

The cycle works in three stages. First, organic waste (uneaten food, fish waste, decaying matter) breaks down into ammonia (NH3/NH4+). Ammonia is extremely toxic to fish and corals — even trace amounts (0.25 ppm) can cause gill damage, stress, and death [4]. Second, a group of nitrifying bacteria called Nitrosomonas convert ammonia into nitrite (NO2−), which is equally lethal at any detectable level [3] [4]. Third, a second group of bacteria called Nitrobacter (and Nitrospira) convert nitrite into nitrate (NO3−), which is relatively harmless at low levels [1] [4].

For reef aquariums with corals, nitrate should be kept below 5 ppm [1]. Fish-only tanks can tolerate higher levels (under 40 ppm), but in a reef environment, elevated nitrate fuels nuisance algae growth and can stress sensitive corals [4] [13].

Key Principle: Ammonia and nitrite are lethal at ANY detectable level. The goal of cycling is to establish enough bacteria to process these compounds to zero within hours of being produced. Nitrate is the “safe” end product, removed by water changes or denitrification. [1] [4]
Ammonia (NH3) → Nitrosomonas → Nitrite (NO2−) → Nitrobacter → Nitrate (NO3−)

2. Six Cycling Methods Compared

There are multiple ways to cycle a reef tank, ranging from a few days to several weeks. The fastest methods use concentrated live bacteria products to seed the tank instantly [2] [7]. Here is how they compare:

Method Time Dosing Difficulty Source
Fritz TurboStart 900 5–7 days 1 oz per 25 gal Easy [7]
Dr. Tim’s One & Only + Ammonia 7–14 days 4 drops/gal = 2 ppm NH3 Easy [5] [6]
Seeded Filter Media 0–7 days Transfer from established tank Moderate [3] [11]
Fishless Ammonia Only 3–6 weeks Pure ammonia to 2–4 ppm Moderate [8] [12]
Live Rock (Uncured) 4–8 weeks Die-off provides ammonia Slow [1] [11]
Fish Food / Dead Shrimp 6–8 weeks Decomposing organic matter Slow [12] [15]
Our recommendation: For most reef keepers, Dr. Tim’s One & Only + ammonium chloride or Fritz TurboStart 900 are the fastest, most reliable methods. Both provide live nitrifying bacteria that are ready to work immediately, cutting cycle time from weeks to days. [2] [5] [7]

3. Step-by-Step: Fishless Cycling with Bacteria

Fishless cycling is the gold standard for establishing the nitrogen cycle without subjecting any fish to toxic ammonia and nitrite [5] [8]. Here are the two most popular bacterial product protocols:

Dr. Tim’s Protocol [5] [6]

1

Day 1: Add Bacteria + Ammonia

Fill tank with saltwater at 1.025 SG. Turn OFF protein skimmer and UV sterilizer for 48 hours [5]. Add Dr. Tim’s One & Only (full bottle for tank size). Then add Dr. Tim’s ammonium chloride: 4 drops per gallon to reach approximately 2 ppm ammonia [6]. Do not exceed 5 ppm — high ammonia inhibits the bacteria you just added [5].

2

Day 3: First Test

Test ammonia and nitrite. If both are below 1 ppm, re-dose ammonia back to 2 ppm [6]. If ammonia is still above 1 ppm, wait and test again on day 5. You should start seeing nitrite appear, which means the first group of bacteria (Nitrosomonas) is working [5].

3

Day 6: Second Re-dose

If both ammonia and nitrite are below 1 ppm, re-dose ammonia to 2 ppm again [6]. Nitrite may spike high during this period — this is normal and expected. Do NOT do a water change to lower nitrite; let the bacteria handle it [5].

4

Completion Test

The cycle is complete when you can dose 2 ppm ammonia and see both ammonia AND nitrite drop to <0.2 ppm within 24 hours [5] [6]. At this point, do a large water change (50%+) to bring nitrate down, then you are ready for livestock [6].

Fritz TurboStart 900 Protocol [7]

1

Day 1: Add Bacteria

Fill tank with saltwater at 1.025 SG. Turn OFF UV sterilizer and protein skimmer for 5 days [7]. Add Fritz TurboStart 900 at the rate of 1 oz per 25 gallons [7]. Add an ammonia source (Fritz ammonia or fish food) to feed the bacteria.

2

Days 2–5: Monitor

Test daily. Ammonia and nitrite should begin dropping. Fritz claims their refrigerated bacteria are “true nitrifiers” that begin working immediately [7]. Keep the tank temperature between 75–80°F for optimal bacterial growth [2].

3

Days 5–7: Completion

By day 5–7, ammonia and nitrite should both read 0 ppm. Perform the same completion test: dose 2 ppm ammonia and confirm it processes to 0 within 24 hours [7]. Do a water change to reduce accumulated nitrate, then begin stocking slowly [2].

Critical: UV sterilizers and protein skimmers will remove or kill the live bacteria you just added. Turn them OFF for the entire cycling period — 48 hours minimum (Dr. Tim’s) or 5 days (Fritz). [5] [7]

4. Testing During the Cycle

Monitoring your water parameters during the cycle tells you exactly where you are in the process. You need to test three things: ammonia, nitrite, and nitrate [1] [4].

What to Test

  • Ammonia (NH3) — test every 2–3 days [5]
  • Nitrite (NO2−) — test every 2–3 days [5]
  • Nitrate (NO3−) — test weekly [1]
  • pH — optional during cycling, will fluctuate [13]

Recommended Test Kits

  • API Saltwater Master Kit (budget) [8]
  • Salifert Ammonia & Nitrite (accurate) [12]
  • Hanna Checkers (digital precision) [8]
  • Red Sea Marine Care Kit [12]

What Readings Mean at Each Stage

Stage Ammonia Nitrite Nitrate What’s Happening
Day 1–3 High (2–4 ppm) 0 ppm 0 ppm Bacteria colonizing, no conversion yet [4]
Day 3–7 Dropping Rising Trace Nitrosomonas active — ammonia → nitrite [3]
Day 7–14 Low/0 Peaking then dropping Rising Nitrobacter catching up — nitrite → nitrate [4]
Cycled 0 ppm 0 ppm <5 ppm Full bacterial colony established [5] [6]
Do not let ammonia exceed 5 ppm. High ammonia concentrations actually inhibit the nitrifying bacteria you are trying to grow, stalling or killing the cycle [5] [8]. If ammonia climbs above 5 ppm, do a partial water change to bring it down to 2–3 ppm.

5. The Ugly Phase

Even after your tank is “cycled” (ammonia and nitrite at zero), you will face a period known as the ugly phase — a succession of nuisance algae blooms that every new reef tank goes through [9]. This is normal, expected, and temporary. The total ugly phase typically lasts 3–6 months [9].

Phase Timeline Appearance Management
Diatoms Weeks 1–4 Brown dusty film on glass, rocks, sand Wipe glass; will resolve on its own as silicates deplete [10]
Cyanobacteria Weeks 3–6 Red/maroon slimy sheets on rock and sand Increase flow; siphon during water changes; reduce feeding [9]
Green Hair Algae Weeks 4–8 Green hairy tufts on rock surfaces Manual removal; reduce photoperiod; add CUC (snails, hermits) [9]
Dinoflagellates Weeks 4–12 Brown/gold stringy slime with air bubbles Most difficult; increase biodiversity, UV sterilizer, maintain nutrients [9] [10]
Patience is key. Reef Builders advises: “the ugly stage is a natural part of a reef tank maturing” and attempting to fight it aggressively with chemicals often makes things worse [9]. The best approach is consistent maintenance (water changes, manual removal) and allowing your tank’s microbiology to establish itself over 3–6 months. Your clean-up crew is your best ally during this period [9] [10].

Ugly Phase Survival Tips

  • Don’t panic. Every successful reef tank went through this phase [9]
  • Don’t blast with chemicals. Algaecides disrupt the microbiome you’re trying to build [9]
  • Keep up water changes. 10% weekly during ugly phase [9]
  • Run your lights on a reduced schedule (6–8 hours) until algae subsides [9]
  • Add a diverse clean-up crew once the tank is cycled: Trochus snails, Cerith snails, Nassarius snails, blue-leg hermit crabs [9]
  • Diatoms feed on silicates — ensure your RO/DI membrane is producing 0 TDS water; old membranes leach silicates [10]

6. When to Add Livestock

Once your cycle is confirmed complete (ammonia and nitrite at 0 after dosing 2 ppm ammonia), it’s time to start stocking — but slowly. The bacterial colony you’ve built can only handle a bioload similar to what it was processing during cycling [2] [14]. Adding too much livestock at once can crash the cycle by overwhelming the bacteria [3].

Recommended Stocking Order

1

Clean-Up Crew (Immediately After Cycle)

Start with snails (Trochus, Cerith, Nassarius), hermit crabs, and possibly a sea cucumber. These handle the diatom/algae blooms that follow cycling and produce minimal bioload [9] [14].

2

Hardy Fish (1–2 Weeks After Cycle)

Add 1–2 hardy fish: clownfish, chromis, or royal gramma. These tolerate minor parameter fluctuations and help build up the bioload gradually [2] [14]. Wait 2+ weeks before adding more fish.

3

Soft Corals (2–3 Months)

Zoanthids, mushroom corals, Kenya trees, and leather corals are forgiving of the minor parameter swings a young tank experiences [14]. These are a great confidence builder before moving to more demanding species.

4

LPS Corals (3–4 Months)

Hammer corals, torch corals, frogspawn, and brain corals need stable parameters and good flow. Wait until your tank has been running smoothly for 3–4 months with consistent alkalinity and calcium [13] [14].

5

SPS Corals (6+ Months)

Acropora, Montipora, Stylophora, and other SPS corals are the most parameter-sensitive livestock in the hobby. They require rock-solid stability in alkalinity (±0.3 dKH daily), calcium, and magnesium [13]. Most experienced reefers recommend waiting at least 6 months, ideally 9–12, before attempting SPS [14].

The #1 mistake new reefers make is adding too much too fast. Add no more than 1–2 fish per 2-week period to allow the bacterial colony to grow proportionally to the bioload [2] [3]. An ammonia spike 2 weeks after “completing” the cycle almost always means you overstocked too quickly.

7. 13 Common Cycling Mistakes

These mistakes account for the vast majority of cycling failures. Avoid them and your cycle will go smoothly:

1. Adding fish before the cycle is complete

This is the most common and most harmful mistake. Fish produce ammonia continuously, and an uncycled tank cannot process it. The result is ammonia burns, gill damage, disease, and often death [1] [3].

2. Letting ammonia exceed 5 ppm

Concentrations above 5 ppm inhibit nitrifying bacteria, stalling or killing the cycle [5] [8]. If using the “dead shrimp” method, remove the shrimp once ammonia reaches 3–4 ppm.

3. Running a protein skimmer during bacterial dosing

Protein skimmers remove organic molecules from the water — including the live bacteria you just added. Turn off the skimmer for at least 48 hours (Dr. Tim’s) or 5 days (Fritz) after adding bacteria [5] [7].

4. Running UV sterilizer during cycling

UV sterilizers kill bacteria passing through them — including nitrifying bacteria in the water column. Keep UV off until cycling is complete [5] [7].

5. Doing water changes during the cycle

Water changes dilute the ammonia and nitrite that the bacteria need as food. Unless ammonia exceeds 5 ppm (toxic to the bacteria themselves), do NOT do water changes during the cycle [5] [8].

6. Using expired or poorly stored bacteria products

Live bacteria products must be kept refrigerated (Dr. Tim’s and Fritz both require this). Expired bottles or those left in a hot warehouse may contain dead bacteria and do nothing [5] [7]. Check the expiration date and buy from reputable retailers.

7. Using “ammonia” with surfactants

Only use pure ammonia or ammonium chloride. Household ammonia from the cleaning aisle often contains detergents or fragrances that are lethal to nitrifying bacteria and tank life [5] [8]. Dr. Tim’s and Fritz both sell reef-safe ammonium chloride.

8. Panicking at the nitrite spike

A massive nitrite spike (often reading off the chart at 5+ ppm) is completely normal mid-cycle. It means the first stage of bacteria is working. Do not intervene — the second stage (Nitrobacter) will catch up [4] [13].

9. Not providing an ammonia source

Bacteria need food. Adding bottled bacteria without an ammonia source means the bacteria will starve and die within days [5] [6]. Always pair bacteria products with ammonium chloride drops.

10. Testing inaccurately

Old test kits, expired reagents, or poorly calibrated digital meters give false readings. A false “0 ammonia” reading leads people to add fish too early [8] [12]. Replace test kit reagents annually.

11. Cycling without flow or heat

Nitrifying bacteria need oxygenated water and warm temperatures (75–80°F / 24–27°C) to grow efficiently [2] [11]. A stagnant, cool tank cycles much slower. Run your return pump and heater throughout cycling.

12. Giving up too soon

Without bottled bacteria, a natural cycle takes 4–8 weeks [11] [12]. Many beginners get impatient at week 2 and either add fish (mistake #1) or tear down the tank. The cycle will complete — be patient.

13. Overstocking immediately after the cycle completes

The bacterial colony is sized to the ammonia load it processed during cycling. Adding 10 fish at once produces far more ammonia than the colony can handle, causing a “mini-cycle” crash [2] [3] [14]. Stock slowly: 1–2 fish every 2 weeks.

8. Frequently Asked Questions

How long does it take to cycle a reef tank?

With bottled bacteria (Fritz TurboStart or Dr. Tim’s One & Only), 5–14 days [5] [7]. With seeded filter media from an established tank, 0–7 days [3]. With just live rock and no bacteria, 4–8 weeks [1] [11]. Using fish food or a dead shrimp as the ammonia source takes the longest: 6–8 weeks [12] [15].

Can I cycle a reef tank with fish in it?

Technically yes (“fish-in cycling”), but it is not recommended. Fish-in cycling exposes your fish to toxic ammonia and nitrite for weeks, causing stress, gill damage, weakened immune systems, and often death [1] [3]. Fishless cycling with bottled bacteria is faster, safer for livestock, and costs less than replacing dead fish [5].

How do I know when cycling is complete?

The cycle is complete when you can dose 2 ppm ammonia and see both ammonia AND nitrite drop to <0.2 ppm within 24 hours [5] [6]. Simply reading “0 ammonia” once is NOT enough — you must confirm the bacteria can handle a full dose. Nitrate should be present (it’s the end product), which you reduce with a water change before adding livestock [6].

Should I do water changes during cycling?

Generally no. Water changes dilute the ammonia and nitrite that bacteria feed on, slowing the cycle [5]. The only exception: if ammonia exceeds 5 ppm, do a partial water change to bring it down to 2–3 ppm, because concentrations above 5 ppm inhibit bacterial growth [5] [8]. After the cycle is complete, do a large water change (50%+) to lower accumulated nitrate before adding livestock [6].

Is the ugly phase the same as cycling?

No. Cycling (nitrogen cycle) takes 1–8 weeks and establishes ammonia/nitrite-converting bacteria [1]. The ugly phase (diatoms, cyano, hair algae, dinoflagellates) starts after cycling and lasts 3–6 additional months as the tank’s full microbial ecosystem matures [9]. You can safely add livestock after cycling completes even though the ugly phase is still ongoing.

Do I need live rock to cycle a reef tank?

Not anymore. Dry rock (like CaribSea Life Rock or Real Reef Rock) combined with bottled bacteria works just as well for cycling [2] [8]. Live rock can speed things up since it comes pre-colonized with bacteria, but it also introduces potential hitchhikers (aiptasia, bristle worms, mantis shrimp) [11]. Many modern reefers prefer dry rock + bottled bacteria for a cleaner start [2].

References

Every factual claim in this guide is cited to its original source. Click any [n] in the text above to jump here.

  1. Bulk Reef Supply — “Cycle Your Saltwater Aquarium”
  2. Bulk Reef Supply — “How to Cycle a Saltwater Tank: Tips to Help You Succeed”
  3. Reefco Aquariums — “How to Cycle an Aquarium”
  4. The Beginners Reef — “Nitrogen Cycle Step by Step Explanation”
  5. Dr. Tim’s Aquatics — “Fishless Cycling Guide”
  6. Dr. Tim’s Aquatics — “Quick Guide to Fishless Cycling with One & Only”
  7. Fritz Aquatics — “FritzZyme TurboStart 900 Saltwater”
  8. Aquarium Store Depot — “Fishless Cycling Guide”
  9. Reef Builders — “How to Manage the Ugly Stage of a New Reef Tank” (2023)
  10. ReefBum — “Eliminate & Prevent Diatoms in Your Reef Tank”
  11. Melev’s Reef — “How to Cycle an Aquarium”
  12. Nano-Reef — “A Guide to Reef Aquarium Cycling”
  13. Reef2Reef — “A Beginner’s Guide to Understanding the Nitrogen Cycle”
  14. The Biota Group — “How to Cycle a Reef Aquarium”
  15. Coralife — “How to Cycle a Saltwater Tank: A Guide for Newbies”

Ready to build your reef?

Browse thousands of coral species from trusted Thai reef shops.

Browse Coral Collection