Finfern honest freshwater care

Free tool

Cycle Coach

Cycling a new tank is a waiting game with three stages — and it's hard to tell where you are. Log each water test and the coach tells you which stage you're in, exactly what to do next, when to test again, and the day it's finally safe to add fish. Set a test reminder so you never lose track. Your log saves in this browser.

You're cycling:

Add a reading

Your data stays here. 🔒 Your cycle log is saved on this device only — never uploaded. How we handle your data → New to cycling? Read the nitrogen cycle first. The completion test: your tank turns a ~2 ppm ammonia dose into 0 ammonia + 0 nitrite within 24 hours.
📅 Never forget to test. The #1 reason a cycle stalls — or fish die in a fish-in cycle — is simply forgetting to test. Add a recurring "test my cycling tank" reminder (every 2 days) to your own calendar; it links back here to log the reading. Nothing is uploaded.

📅 Cycled? Keep it healthy. Once your tank is cycled, the job becomes a steady routine — add recurring weekly water-change & test and monthly filter-maintenance reminders straight to your own calendar. Nothing is uploaded; the events just live in your calendar and link back here.

The three cycle stages at a glance

Cycling grows two groups of bacteria in sequence. These are the stages the tracker reads from your logged ammonia, nitrite and nitrate — enter your own numbers above to see exactly where you are.

StageYour readings look likeWhat it means
Stage 1 — Ammonia phase Ammonia present, nitrite still 0 The first bacteria are establishing. Keep dosing/feeding lightly and testing.
Stage 2 — Nitrite phase Ammonia converting to 0, nitrite high The second group of bacteria is catching up. This is the longest, most frustrating stage — keep testing.
Stage 3 — Nitrate appearing Nitrite falling, nitrate showing Nitrate is being produced and nitrite is on its way down. The cycle is almost complete.
Cycled 🎉 Ammonia 0, nitrite 0, nitrate present Your bacteria process waste fully. Confirm with the completion test, then do a big water change and add fish a few at a time.

The completion test: dose ~2 ppm ammonia and it should read 0 ammonia + 0 nitrite within 24 hours. Any ammonia or nitrite above 0 is harmful; keep nitrate under 50 ppm with water changes. New to this? Read the nitrogen cycle.

Taking a while? Speed it up — and why cycles stall

🚀 Speed it up
  • Seed it with live bacteria. A used filter sponge, mature media, or a cup of gravel from an established, healthy tank is the real accelerator — or dose a bottled nitrifying-bacteria starter.
  • Keep it warm. Nitrifying bacteria grow fastest around 26–29 °C, so a heater shortens the cycle.
  • Feed them steadily. Hold the ammonia around 2 ppm — too little starves the bacteria, far too much (over ~5 ppm) stalls them.
  • Don't clean it. No water changes and no filter rinsing mid-cycle — you'd wash out the bacteria you're growing.
🧯 Why it stalls
  • The pH crashed. Nitrifying bacteria slow right down in soft, acidic water and can stall below about pH 7 — check your KH/buffer if numbers stop moving.
  • Chlorine killed them. Un-dechlorinated top-up water, or rinsing media under the tap, wipes out the colony. Always dechlorinate; rinse media in old tank water only.
  • Too cold. A cool tank cycles painfully slowly — check the heater.
  • Ammonia too high. Over ~5 ppm actually inhibits the bacteria — do a water change to dilute it back to ~2 ppm.

Cycling questions

How do I know when my aquarium is cycled?

Your tank is cycled when ammonia reads 0, nitrite reads 0, and nitrate is present — that means both groups of bacteria are processing waste fully. Confirm it with the completion test: dose the tank to about 2 ppm ammonia and it should read 0 ammonia and 0 nitrite within 24 hours. Once it passes, do a big water change to lower the nitrate, then add fish a few at a time.

What are the stages of the nitrogen cycle?

There are three. Stage 1 (ammonia phase): ammonia is present but nitrite is still 0 as the first bacteria establish. Stage 2 (nitrite phase): ammonia is converting to 0 but nitrite is high — this is the longest, most frustrating stage. Stage 3 (nitrate appearing): nitrite falls and nitrate shows up, meaning the cycle is nearly complete. You're cycled when ammonia and nitrite are both 0 with nitrate present.

What ammonia and nitrite levels are safe for fish?

Any ammonia or nitrite above 0 is harmful — both are toxic to fish. If you have fish in the tank and either reads above 0, do a water change now to bring them down, feed lightly, and don't add more fish until you're cycled. Nitrate is far less toxic; keep it under 50 ppm with regular water changes.

Can I add fish while the tank is still cycling?

Not into a fishless cycle — wait until it's finished (ammonia 0, nitrite 0, nitrate present), then do a big water change and add fish gradually. If fish are already in the tank (a fish-in cycle), don't add any more until it's cycled: keep ammonia and nitrite down with water changes and feed lightly, because any reading above 0 is harmful.

Why is my aquarium cycle stuck or taking so long?

The most common cause is a pH crash — nitrifying bacteria slow right down in soft, acidic water and can stall below about pH 7, so check your KH/buffer if the numbers stop moving. Other stalls: chlorinated top-up water or rinsing filter media under the tap (both kill the bacteria — always dechlorinate and rinse media in old tank water only), a tank that's too cold, or an ammonia level over about 5 ppm, which actually inhibits the bacteria (dilute it back toward 2 ppm). To speed a cycle up, seed it with mature filter media or a bottled nitrifying-bacteria starter, keep it warm (about 26–29 °C), and hold the ammonia around 2 ppm.