My tank is cycled — now what do I do first?

Short answer: Before any animals go in, do a large (50–75%) water change to drop the nitrate that built up during cycling, then confirm your parameters are stable for about a week. Add life slowly and in order: clean-up crew first, then a hardy fish or two, then corals over the following months. Never dump in a full stock at once.

The details — the first-steps checklist

1. Big water change. Cycling leaves nitrate high. Change 50–75% of the water to bring it down before livestock. Verify nitrate is under 20 ppm (ideally under 10 for a reef).

2. Confirm the environment is stable. Temperature 76–80°F, salinity 1.024–1.026 sg, pH 8.0–8.3, and ammonia/nitrite still 0. Watch it hold for about a week before adding animals — a freshly-finished cycle that's proven stable survives its first residents; a rushed one doesn't. (This is the gate to leave the cycling stage.)

3. Add the clean-up crew first. Snails and hermit crabs are your first livestock. They're hardy, they graze the diatom/algae blooms of the ugly phase, and they add almost no bioload.

4. Then a hardy fish or two. Clownfish, chromis, or cardinalfish are classic firsts. Add only 2–3 at a time, then wait 1–2 weeks before the next addition so the bacteria can grow to match the new load. (Quarantining new fish in a separate tank first prevents disease outbreaks — worth doing from your very first fish.)

5. Corals come later, easiest first. Soft corals after ~2–3 months of stability, LPS after ~3–4 months, SPS after 6+ months. Add least-aggressive species before aggressive ones.

The numbers

Common mistakes

• Fully stocking on day one. The colony grows to match the bioload gradually; a flood of fish overwhelms it and you get an ammonia spike in a "cycled" tank.
• Skipping the pre-livestock water change. High leftover nitrate stresses the first animals.
• No quarantine. One un-quarantined fish can introduce ich or velvet and wipe the tank.

When to worry

• Normal: The ugly phase kicking in right as you add the clean-up crew — that's expected; see the-ugly-phase-is-normal.
• Worry: Any ammonia or nitrite reappearing above 0 after you add livestock = you added too much too fast. Stop adding, do a water change, and let the colony catch up.

What's next

You've cleared the cycling stage. Next stop is the first-livestock stage — start with cleanup-crew-what-to-add-first, then choosing, acclimating, and quarantining your first fish.