The nitrogen cycle

A brand-new aquarium is not ready for fish on day one. The filter is a box of wet foam. It does not process waste until bacteria colonize the media. Until then, fish pee ammonia into a closed box.

The three numbers

Clear water is not a cycle. A tank can look like a magazine and still have 2 ppm ammonia.

Use a liquid test kit. Strips miss ammonia on a new tank often enough that people add fish into a spike.

Full parameter context: water parameters.

What “cycled” means

Both of these, on consecutive tests a day or two apart:

1. Ammonia 0
2. Nitrite 0
3. Nitrate present (not stuck at 0 forever)

If ammonia and nitrite are 0 and nitrate is also 0 after weeks of “cycling,” you either never added an ammonia source, or plants/water changes are removing nitrate faster than you can see it. Dose a known amount of ammonia, wait 24 hours, and test again. A cycled filter knocks 2 ppm ammonia to 0 within a day.

Fishless cycle (do this)

1. Set the tank: dechlorinated water, filter running 24/7, heater at ~78°F. Bacteria grow faster warm.
2. Add a pure ammonia source (no surfactants) to about 2–4 ppm, or a pinch of fish food every other day — pick one method and stick to it.
3. Test every day or two. You will see ammonia climb, then nitrite climb as ammonia falls, then both fall while nitrate appears.
4. When ammonia and nitrite stay at 0 after you dose, you are cycled.
5. Do a large water change to bring nitrate down. Then add a small first group of fish. Not the full stock list.

Plan on 3–6 weeks. Bottled bacteria can shorten that if the bottle is alive and you still test. It is not a 24-hour product. If someone sells a cycle in a day, they are selling the bottle.

Seed from an established tank if you can: a squeeze of dirty sponge in a bag of tank water beats starting from zero. Do not rinse that sponge in tap water.

Fish-in cycle (avoid this)

You can keep fish alive through a cycle with daily tests and large water changes every time ammonia or nitrite is readable. It is slower for the bacteria and worse for the fish. There is no prize for starting with livestock.

If you already have fish in an uncycled tank: test daily, change water when ammonia or nitrite is above 0, do not feed much, do not add more fish. Details on gasping: fish at the surface.

After it is cycled

The colony lives on the filter media and other surfaces. It needs flow and a little waste. It dies if you:

• Rinse all the media under the tap on the same day
• Let the filter sit dry for hours
• Bleach the sponge “to be thorough”

Squeeze mechanical fluff in a bucket of old tank water. Leave biological media alone unless it is actually clogged.

Keep bioload inside what that colony can handle with the stocking calculator. Feed what disappears in two minutes — leftover flakes become ammonia again. Weekly water changes remove nitrate; plants help, they do not replace the siphon.

A boring timeline

Your tank will not match this calendar. Test. The calendar is a rumor; the kit is the job.