The nitrogen cycle explained
A new fish tank can kill fish faster than an old one. The reason is the nitrogen cycle, and most beginner problems come from not understanding it.
This piece covers what's happening biologically, how to know your tank is cycled, and what to do when something goes wrong.
What ammonia is and where it comes from
Fish breathe out ammonia. They also pee it out, and uneaten food rots into it. So does dead plant matter, dead fish, and pretty much anything organic that breaks down in water.
Ammonia is poisonous to fish at very low levels. Even 0.25 ppm causes stress. At 1 ppm it starts doing real gill damage. Above 2 ppm and you're looking at deaths.
In a brand new tank with no biological filtration, ammonia just accumulates. This is why "new tank syndrome" exists and why dropping fish into uncycled water is a slow kill.
The bacteria do the work
Two groups of bacteria handle this:
- Nitrosomonas (and similar species) eat ammonia and excrete nitrite
- Nitrobacter and Nitrospira eat nitrite and excrete nitrate
Nitrite is also poisonous to fish (it binds to hemoglobin and stops oxygen transport, the fish version of carbon monoxide poisoning). Nitrate is only mildly toxic and accumulates slowly. You remove it with water changes or, in a planted tank, plants take it up directly.
The full chain looks like:
fish waste -> ammonia (NH3) -> nitrite (NO2) -> nitrate (NO3) -> water change
A "cycled" tank means you've grown enough of both bacteria types that they keep up with the fish load. Ammonia and nitrite should both read zero. Nitrate slowly climbs until you change water.
How long it takes
Four to eight weeks for most tanks. Faster with bottled bacteria (Tetra SafeStart, Seachem Stability, Dr. Tim's). Slower in cold water. Stalls completely if pH drops below about 6.
The bacteria live mostly in your filter media, not your water. This is why you never replace all your filter media at once and why filter cartridges that say "replace monthly" are scammy. Rinse, don't replace.
Fishless cycling
The cleanest way to cycle a new tank: dose ammonia (Dr. Tim's Ammonium Chloride works) to about 2 ppm, wait for nitrite to spike then drop, keep dosing until ammonia and nitrite both convert to zero within 24 hours of a fresh dose. Then water change down to safe nitrate levels and add fish.
Takes about a month with patience. No fish suffering.
Fish-in cycling
If you already have fish in there (you bought a tank kit and the store sold you fish the same day, this happens constantly), you cycle with the fish in. This means:
- Test ammonia and nitrite daily
- Water change immediately when either climbs above 0.25 ppm
- Don't feed heavily
- Use a binder like Prime to detoxify ammonia between water changes
It works, but it's stressful for the fish.
Knowing it's cycled
A test kit (API Master Test Kit is the standard) showing ammonia 0, nitrite 0, nitrate detectable. The nitrate is the proof. Without that final number climbing, you haven't actually grown the second bacteria group yet.
Liquid test kits, not strips. Strips lie.
What breaks a cycle
- Replacing all filter media at once
- Bleaching your filter
- Running medications that kill bacteria (most antibiotics will)
- Letting the filter run dry for hours
- Big swings in pH
If you nuke your cycle, you go back to mini-cycling: small ammonia spikes, frequent water changes, a couple weeks to rebuild. Less drama than the first cycle because the rest of the tank already has bacteria seeded in the substrate and decor.
Why this matters for planted tanks and aquaponics
In a planted tank, plants take up ammonia directly (they prefer it to nitrate). Heavily planted tanks with low fish loads can run with very small bacterial colonies because the plants get to the ammonia first.
In aquaponics, the whole point is that nitrate feeds your plants. You want a fully cycled system so you have a steady nitrate supply. Aquaponics also uses a third group, the nitrate-reducing bacteria that turn excess nitrate into nitrogen gas. Most home systems don't get this third stage going.