You have the tank, the dry rock, the sand and the return pump. What you do not have is a filter. Live rock arrives part-built, carrying nitrifying bacteria and all the microfauna that comes with it. Dry rock is clean mineral — no bacteria, no life, no shortcuts. So you build the biofilter yourself, seeded from a bottle, and the calendar works out at roughly a month if you feed it properly and test rather than guess.
Why dry rock cycles differently
The bacteria that turn ammonia into nitrite, and nitrite into nitrate, need two things: surfaces to colonise and a steady supply of ammonia. Dry rock gives you plenty of the first. It gives you none of the second, which is why dropping a bottle of bacteria into an empty tank with no ammonia source achieves very little. The bacteria die off within days and you start again.
Dry rock also tends to arrive with bound phosphate and dried organic matter in its pores, particularly the mined varieties. Soaking new rock in RO water with a powerhead for a week or two, changing the water a couple of times, reduces the algae bloom that often follows a dry-rock cycle. It is not compulsory, but it makes month two much easier.
What you need before day one
- RO/DI water mixed to 35 ppt (around 1.025–1.026 SG), verified with a refractometer
- Heater, powerhead or two, and your sump running if you have one
- Test kits for ammonia, nitrite, nitrate, pH and alkalinity — liquid kits, not strips
- A bottled nitrifying bacteria product, kept cool
- An ammonia source: ammonium chloride solution is the cleanest option
- Lights off, or on a short low-intensity photoperiod only
Getting the bottled bacteria right
Bottled products are live cultures, and they are more fragile than the label suggests. Shake the bottle, dose to the instructions for your water volume, and switch off the protein skimmer, UV steriliser and ozone for three or four days afterwards — UV will kill what you have just paid for, and a skimmer will strip it out of the water column. Keep the tank at 25–26°C and the salinity correct.
Add sand and rock first, let the dust settle, then dose. Many reefers dose a second time around day seven, or after the first ammonia drop, to top up the population while the biofilm establishes. That is cheap insurance rather than a requirement.
The 30-day testing schedule
Week 1: feed the cycle
Day one is plumbing, mixing, aquascaping and dosing. Add ammonium chloride to bring the reading to 1–2 ppm, then test daily. Ammonia should start falling by day three or four. Nitrite usually appears right at the end of the week. Write every result down — the direction of travel matters more than any single number.
Week 2: ammonia down, nitrite up
Ammonia should now be at or near zero, and nitrite will be climbing, sometimes off the scale of a hobby kit. Nitrate starts to register. Each time ammonia hits zero, dose it back to 1 ppm. Skipping this step starves the bacteria you are trying to grow, and the cycle stalls.
Week 3: nitrite peaks and falls
This is the week the nitrite-oxidisers catch up. Expect a brown diatom film on the glass and sand, and possibly a little green algae. Both are normal, and both are easier to manage with the lights still low. Leave the livestock out. If nitrate climbs very high, a 20–30% water change at the end of the week is fine.
Week 4: prove it works
Dose ammonia to 1–2 ppm and test 24 hours later. A finished biofilter clears ammonia to zero and nitrite to zero within 24–48 hours, leaving nitrate behind. Do a decent water change, then add your first small fish, or a cleanup crew if you prefer to move more slowly.
Reading test results without panicking
Ammonia kits can give false positives if you are using an ammonia-binding conditioner, which is a common trap in the first fortnight. Nitrate kits read falsely high while nitrite is still present, so do not chase that number yet. Nitrification also consumes alkalinity, so check dKH through the cycle and buffer if it slips below about 7. And read the instructions every time — the shake times on nitrate kits are there for a reason.
Mistakes that stretch the cycle
- Using a fish to provide ammonia. It is unpleasant for the fish and gives you far less control.
- Running UV or a skimmer during the first week of seeding.
- Full lighting from day one, which simply grows algae while you wait.
- Big water changes during the ammonia phase, which slow the process rather than speed it up.
- Adding five fish the day the cycle finishes. The biofilter is matched to the ammonia you fed it, not to a shoal.
Day 30 and beyond
A cycled tank is not a mature tank. Stock in small stages over the following weeks, quarantine what you can, and feed lightly for the first fortnight after each addition. Test ammonia and nitrite weekly for the first couple of months; if either registers, a water change and a dose of bacteria will usually sort it before livestock suffers. Adding a small piece of live rock or a bag of live sand later will bring in coralline algae and microfauna, though neither is needed to cycle.
Thirty days is a guide, not a rule. Well-fed dry-rock systems occasionally finish inside three weeks; cold tanks and under-dosed bottles can take six. Trust the tests, keep the log, and be patient at the point where the nitrite refuses to move. It always does.
Photo: WikiImages / Pixabay
