The single biggest predictor of whether someone is still in this hobby in two years is how they spent their first eight weeks. Cycling is not an exciting phase, and almost every expensive disaster in reefkeeping traces back to rushing it.
What cycling actually is
You are growing a bacterial population, not treating water. Two groups matter: bacteria that convert ammonia to nitrite, and bacteria that convert nitrite to nitrate. Until both are established in sufficient numbers, anything you add to the tank is producing a toxin the system cannot process.
That is the whole thing. Everything else — the products, the schedules, the arguments — is detail around growing bacteria.
The sequence
- Set up and run everything. Salt mixed to 1.025–1.026, heater at temperature, pumps and return running. Let the sand settle and the water clear.
- Add an ammonia source. Either dose bottled ammonia to a low, deliberate level, or use live rock that came with organics on it. A pinch of food works but is imprecise. The point is to feed the bacteria something to colonise.
- Test ammonia, nitrite and nitrate every couple of days — and write it down. You are watching for a pattern, and the pattern is the only real evidence of progress.
- Wait for the curve. Ammonia rises, then falls. Nitrite rises as ammonia falls, then falls itself. Nitrate accumulates. When ammonia and nitrite both read zero and nitrate is present, your cycle is complete. Not before.
- Water change, then add slowly. One or two hardy fish, or a few cheap corals. Not a stocking list.
Realistically this takes two to six weeks. Bottled bacteria products can genuinely shorten it, but they do not eliminate the need to verify with tests. The bottle is not the proof; zero ammonia and zero nitrite is the proof.
The mistakes that cost the most
Adding livestock during the cycle. “Just one fish to help it along” means that fish sits in ammonia for weeks. It is bad for the animal and it does not speed anything up.
Water changes to fix a high reading mid-cycle. High ammonia during a cycle is the cycle working. Diluting it away removes the food source and prolongs the process. Leave it alone.
Trusting the calendar over the test kit. Tanks are not done at four weeks because four weeks passed. They are done when the numbers say so, and tanks vary enormously.
Buying corals in week three. A new tank is chemically unstable and will stay that way for a while after the cycle finishes. Give it a month of boring stability before you put anything expensive in it.
Skipping quarantine because the tank is new. This is the one that stings. A brand new tank is your one chance to have a system with no pests in it at all. Every coral you drop in undipped and un-inspected during those first months is a hitchhiker you will be fighting for years. Start the dip-and-inspect habit with the very first frag.
What to expect after the cycle
A diatom bloom — brown gritty dusting on sand and rock — is normal in the weeks after cycling and goes away on its own. Do not treat it. Some cyano is common too. New-tank ugly phase is real, it is temporary, and the reefers who panic and start dosing things during it are the ones who end up with genuine problems.
Start with the numbers right
Our free reef water chemistry chart gives you every target range on one printable page — useful from day one, and it costs nothing.
The Reef Water Parameters Quick Reference Guide covers what each parameter does and how to correct it safely, and The Complete Coral Dipping Guide gets the pest-prevention habit right from your first coral, which is the cheapest insurance in the hobby.