How Long to Cycle a Fish Tank: 4–6 Weeks, or Days if Seeded

The short version

How long you wait depends mostly on whether the filter starts with bacteria in it.

Starting from scratch, the bacteria take 30 to 45 days to establish on their own 1. Aquarium Science, a hobbyist site run by one experienced keeper, puts it at 2 to 8 weeks, usually 4 to 6 4. Plan on a month to six weeks. Putting fish in doesn’t speed it up (our guide to cycling with fish in the tank covers how to do it safely). It’s still about 30 days unless you add an effective bacteria product 1.

Borrow media from a tank that’s already running and it can drop to 4 to 12 days 5.

Why it takes weeks

Both groups of bacteria, the ones that eat ammonia and the ones that eat nitrite, grow slowly 1. The cycle runs in two waves. Without bottled bacteria, ammonia typically drops below 1 mg/L around day 9 to 12, and nitrite doesn’t peak until somewhere between days 14 and 20 1. After the nitrite peak, it keeps dropping until it levels off 3.

That second wave is where most of the waiting happens. If you’re watching nitrite climb in week three, the cycle isn’t failing. It’s on schedule.

Seeding with an established filter

Nothing else saves as much time. Filter “gunk” from an established tank can cut the cycle from 4 to 6 weeks to 4 to 12 days 5. Fish farms do the same thing, and consider moving media from a system that’s already running under similar conditions the most effective way to add bacteria 3.

A few conditions apply 5:

• “Established” means the media has been in a filter for at least two months.
• Biological media or gravel from an under-gravel filter works much better than gunk from a mechanical filter pad.
• Put it where water flows through it, in the filter, not on the bottom of the tank.

Ask a friend with a healthy tank, or a local fish store you trust. But anything you borrow can carry disease along with the bacteria. That risk comes with every seeding method 3.

Bottled bacteria: the sources disagree

The sources split on this one.

• Dr. Tim’s says its One & Only bacteria, used with its ammonium chloride, “will cycle the tank in less than one week” 1. Its recipe has you test on day 8 and add fish on day 9 if ammonia and nitrite are both under 0.5 ppm 2, which is already a day past the one-week claim. Both figures come from the company that sells the bacteria and the ammonium chloride. Dr. Tim Hovanec was also Chief Science Officer of Aquaria Inc. (Marineland’s parent company) 9.
• The aquaculture fact sheet says commercial bacteria aren’t essential but can shorten start-up 3.
• Aquarium Science ran three tests on eleven popular products, seven of them repeated twice. The author found no significant difference between bottled bacteria and adding nothing 6. Dr. Tim’s One and Only was the one product with some statistically positive effect, which he says “may be due just to dumb luck” 6. He also says the test wasn’t designed for a scientific journal 6.

Our read: none of this is a peer-reviewed comparison of aquarium products. If you use a bottle, keep testing and go by the numbers, not the day count on the label.

Temperature

Warmth speeds the bacteria up. One fish-farm start-up ran short mainly because the water was 83 °F (28 °C) 3, though that was a commercial system, not a home aquarium. A 1981 paper by Watson and colleagues, quoted on Aquarium Science, gives 77 to 86 °F (25 to 30 °C) as the best range for most strains 7.

On how warm to go, the two sources disagree. The fact sheet says to go no more than 3.6 to 5.4 °F (2 to 3 °C) above the temperature you’ll keep the tank at, to avoid a die-off when you turn it back down 3. The Aquarium Science author runs his cycles at 85 to 90 °F 8. We’d follow the fact sheet if you plan to keep fish that like it cooler.

pH and alkalinity

Low pH is the usual reason a cycle stalls (our aquarium pH guide has the wider picture). Below 7.0 is where most stalls happen 1, and nitrifying bacteria reach their lower healthy limit at about 6.8 3. In one hobbyist test, a pH of 6.0 “stopped cycling cold” 7.

The bacteria make acid as they work, so pH tends to fall during the cycle 1. That’s why one experienced keeper adds baking soda to hold pH at 7.4 8, and why Dr. Tim’s recipe calls for a water change if pH drops below 7 2. The nitrogen cycle guide explains where the acid comes from.

Very soft, acidic tanks are a different case. Archaea, a group of nitrifiers that can work at very low pH, double about every ten days, compared with about every 24 hours for nitrifying bacteria. That’s why a low-pH tank can take four to twenty-four months to cycle 7. The months figure is one hobbyist’s estimate.

How you feed the cycle

How you dose matters too. In one hobbyist comparison, adding ammonia only when it dropped to 0.5 ppm, a regimen he attributes to Dr. Tim’s, took 11 days longer than adding 2 ppm every day 7. Dr. Tim’s current recipe works differently: it doses on days 3 and 6 if levels are under 5 ppm 2. So that result may not apply to it.

For the actual dosing steps, see the fishless cycle guide, and use the fishless cycle calculator to work out how much ammonia your tank needs.

One hobbyist’s fast method

The Aquarium Science author runs his cycles warm, with pH held at 7.4, lots of filter media and ammonia every day. He reports that about half the time the cycle is done after ten days, the other half it takes another week, and it rarely needs four weeks 8. He calls it done when ammonia and nitrite are under 0.25 ppm 8. That’s an experienced keeper’s log, not a controlled study, and he also says the process isn’t very predictable 8.

When to stop counting days

The calendar is only a guide. The cycle is done when the tank can clear a full dose of ammonia to nitrate overnight with no nitrite left 1. With the bottled-bacteria recipe, the cutoff is ammonia and nitrite both under 0.5 ppm 2. Our rule is zero on both, 24 hours after a full dose, as set out in the nitrogen cycle guide. Is my tank cycled? walks through the 24-hour test.

FAQ

How long does it take to cycle a fish tank? Starting from scratch, 30 to 45 days according to Dr. Tim’s Aquatics 1. Aquarium Science says 2 to 8 weeks, usually 4 to 6 4.

How can I make a tank cycle faster? Media from an established tank can cut it to 4 to 12 days 5. “Established” means the media has been in a filter for at least two months, and it has to sit where water flows through it 5. Warmth helps too 3.

Do bottled bacteria work? The sources disagree. Dr. Tim’s says its One & Only bacteria will cycle a tank in less than one week 1. Aquarium Science found no significant difference between bottled bacteria and adding nothing 6.

Does adding fish speed up the cycle? No. It’s still about 30 days unless you add an effective bacteria product 1.

My nitrite is still climbing in week three. Is the cycle failing? Probably not. Without bottled bacteria, nitrite doesn’t peak until somewhere between days 14 and 20 1.

How do I know it’s done? When the tank can clear a full dose of ammonia to nitrate overnight with no nitrite left 1. See Is my tank cycled?.