Reef cycling to first fish

Reef startup is mostly about growing nitrifying bacteria. They settle on rock, sand, and filter media; getting the colony ready for the first fish’s waste usually takes 3–6 weeks. Once ammonia and nitrite read zero, add a small cleanup crew, then wait about two weeks before the first fish.

Why cycle at all?

Fish waste, uneaten food, and coral mucus all end up as ammonia (NH₃ and NH₄⁺). Around 0.1 ppm, gills start to burn; above about 0.25 ppm, fish often cannot hold on. Nitrite (NO₂⁻) follows, and around 0.5 ppm fish look oxygen-stressed and washed out.

Two groups of bacteria push those compounds down the line: one turns ammonia into nitrite, the other turns nitrite into nitrate. Nitrate is far less toxic and leaves with water changes and algae.

The two groups run at different speeds. The first often keeps up in 3–7 days, so the ammonia peak arrives early. The second is slower and often needs 10–20 days, which is why the nitrite peak trails the ammonia peak. During a cycle, test results matter more than the calendar.

A reef sump usually does two jobs: a filter sock or roller traps particles, and the protein skimmer pulls organic matter out before it breaks down into ammonia. It does not handle nitrate. Nitrification itself is carried by the rock and sand, and the colony establishes with no bio-media in the sump at all. The day-by-day version for freshwater is in the first month of a new tank; the bacteria work the same way in both.

What to settle on day one?

A few decisions on day one set how fast the bacteria can grow.

• Water: RO/DI water plus marine salt, at 1.024–1.026 SG (about 35 ppt). Tap water often carries chloramine, copper, and phosphate.
• Rock: start at 1 kg of live or dry rock per 8–10 L of water. Rougher, more porous rock holds more bacteria.
• Flow: a return pump plus a wavemaker, sized to move 10–20 times the tank volume per hour. Flow delivers ammonia to the bacteria and lifts debris out of the crevices.
• Temperature: 24–26 °C, where most heaters hold steadiest.
• Light: off, or 4–6 hours a day during the cycle. Full lighting on rising nitrate tends to grow algae first.
• Test kits: ammonia, nitrite, nitrate, alkalinity, and pH, plus a refractometer.

How do the stages run?

The run below assumes no livestock yet and a bottled or chemical ammonia source. With live rock or animals in the tank, mainly the water-change line changes.

Bottled bacteria can move those numbers up by a few days. They do not replace testing: with the same ammonia going in, the colony still takes the time it takes.

Live rock or dry rock?

Live rock comes out of the sea carrying bacteria, algae, sponges, and hitchhikers. Some of that life dies in transit, so fresh live rock often brings its own ammonia source and needs no dosing.

The usual hitchhikers are aiptasia, bristleworms, mantis shrimp, and small crabs. The first two are mostly a nuisance: aiptasia multiplies fast and takes space coral needs, and peppermint shrimp eat it; bristleworms work the crevices for leftover food. Mantis shrimp are different—they crack snail shells and take small fish and shrimp, so catch them and keep them on their own when you can.

To skip hitchhikers, use dry rock. Dry rock has no hitchhikers and no ready bacteria, so the colony starts from nothing and the cycle runs longer. A common middle route is mostly dry rock, one small piece of fresh live rock to seed it, and a bottled culture—often about 3–4 weeks for a first tank. Full dry rock with an ammonia source commonly runs 4–8 weeks; mostly fresh live rock can land in 2–3 weeks, with hitchhikers coming along.

If live rock spent more than a day in transit, cure it in a separate container for 2–4 weeks: change some water every few days and move it to the tank once ammonia reads zero. Straight into the display, the ammonia from that die-off stays in the tank.

If coral is already in and fish are not, still wait for ammonia and nitrite to read zero. Ammonia burns gills and coral alike.

What to test, and how often?

Testing is the steadiest read on progress during the cycle; once the tank settles, the schedule can loosen.

Evaporation takes water and leaves the salt behind, so top-off water is best kept as RO/DI only. Topping off with saltwater drives salinity up—1.025 to 1.030 inside a week is common, and snails and coral feel it first.

Skip water changes during the cycle unless ammonia or nitrite passes 2 ppm with animals in the tank. Then change about 30% to dilute it, matching temperature and salinity in the new water first.

Why the crew goes in first?

Once ammonia and nitrite have read zero for several days, add a small crew and wait about two more weeks before the first fish. The crew’s waste is small, which gives the colony a stretch at low load to fill in, and helps with early diatoms and detritus.

Numbers follow rock surface area; tank volume is only a rough starting point. Add snails when algae climbs, hermits when detritus collects in the crevices, and nothing when neither is heavy—hungry crew turns to coral and snails.

How to choose the first fish?

The first fish mainly needs two things: eat prepared food, and tolerate a swing in water quality. Color can wait.

Quarantine each fish for 2–4 weeks first. The tank does not have to be large: a bare 20–40 L with a filter and some flow is enough. Watch for white spots, heavy breathing, and scratching against rock. Catching one fish inside a reef tank is close to impossible, so quarantine is usually the cheaper route.

Add one fish at a time. Test ammonia daily for 3–5 days after it goes in: the waste load rises and the colony needs a few days to follow. At the first reading of ammonia, stop feeding and change 20%.

Feed once or twice a day, an amount finished within a few minutes. Leftover pellets and droppings are both ammonia, so overfeeding creates work.

Things to watch

These are the usual mismatches during a cycle, and most turn around inside two weeks.

Three things damage a tank more directly. Best avoided:

• Tap water or a copper-based medication: copper adsorbs into rock and sand and leaches back out, killing snails, shrimp, crabs, and coral first. Tap water also brings chloramine and phosphate, which feed algae.
• Several fish at once: the colony grew to match the last fish’s waste. Add a group and ammonia climbs to a concentration that burns gills. One fish a week looks slow, and is still usually faster than cycling twice.
• A fish straight into the display with no quarantine: parasites such as white spot come with it, catching fish in a reef tank is close to impossible, and the fix left is often 8–11 weeks with the tank fallow.

Checking it in the studio

Magic Reefscape renders a full-tank composition: the rock framework, the coral groups, and where the fish sit are in the picture. The bacteria, ammonia, and nitrite are not in it.

Set biotope, tank size, and framing first, then put the tables on this page onto the real tank. Two things in the render are worth checking: whether the framework carries enough rock—a few isolated stones mean too little surface area to hold fish or a stable colony—and whether the render leaves a sand bed, since 2–3 cm on the bottom is what gives burrowing animals and low-oxygen zones somewhere to be.

The day-by-day ammonia and nitrite work is in the first month of a new tank. Density questions after the first fish go back to the fish table above.

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2026-10-16