The first 90 days of a reef tank follow a pattern: cycle it (usually 3–6 weeks), add a first hardy fish and a clean-up crew, then ride out an “ugly phase” of diatoms and algae while you add livestock slowly. Test often, change one thing at a time, and write everything down. Patience in these months saves most of the problems that come later.
The 90 days at a glance
Every tank runs at its own pace, so treat these as rough stages, not deadlines.
| When | What’s happening | What you do |
|---|---|---|
| Weeks 1–6 | The tank cycles: bacteria grow to process waste | Add an ammonia source, test ammonia and nitrite, don’t add fish yet |
| When cycled | Ammonia and nitrite read zero | A water change, then a first hardy fish |
| Weeks 4–12 | The ugly phase: diatoms, then algae | Wipe, siphon, add a small clean-up crew, don’t panic |
| Weeks 6–12 | The tank settles | Add fish one at a time, try beginner corals |
| Throughout | Habits form | Weekly tests and water changes, logged |
Go slowly
Bulk Reef Supply lists “moving too fast” as the first beginner mistake. A new tank’s bacteria grow with its livestock. Add too much at once and waste outpaces them, which can bring ammonia back. Add one fish at a time, at least two weeks apart, Bulk Reef Supply suggests.
Test more than you think you need to
In the first months, test ammonia and nitrite during the cycle, then nitrate, phosphate, alkalinity and salinity weekly. See which test kits to buy first. The numbers move more in a new tank, and a log shows you the tank settling.
The Companion’s test log takes ammonia and nitrite alongside the usual results, so you can watch the whole cycle on one chart.
Build the routine now
The habits you set in the first 90 days are the ones you keep: a weekly water change, weekly tests, filter socks, glass. Put them on a schedule from the start. See the maintenance checklist and beginner mistakes.
Common questions
How long before I can add fish to a new reef tank?
Once the cycle is complete: ammonia and nitrite at zero. That usually takes 3–6 weeks without bottled bacteria.
When can I add corals?
Most keepers add hardy corals once the tank has cycled and settled for a few weeks, and parameters are steady.
Is the ugly phase normal?
Yes. Reef Builders says every new tank has one.
Sources and limits
Last checked 13 October 2026. This guide brings together the published advice below. Product instructions come first, and an experienced local shop or club can help with anything specific to your tank.
- Bulk Reef Supply: the nitrogen cycle in your aquarium
- Bulk Reef Supply: cycling your saltwater aquarium
- Reef Builders: managing the ugly stage of a new reef tank
- Bulk Reef Supply: top 10 nano reef mistakes
Keep going
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