How to Tell If Your Tank Is Cycled: The 24-Hour Ammonia Test

The 24-hour ammonia test

You can’t tell by looking. Clear water and a few weeks on the calendar prove nothing. The only real check is to give the filter a job and see if it finishes it.

Dose ammonia the way you have been, wait a day and test ammonia and nitrite. The cycle is complete when a full dose of 2 to 3 mg/L (as nitrogen) turns into nitrate overnight with no nitrite left 1. A slightly looser version of the same test accepts ammonia at 0.25 ppm or less 24 hours after feeding 4.

Don’t chase an exact 2 ppm on your kit. It won’t read exactly 2, and it doesn’t need to. What matters is not adding too much 3. The fishless cycle calculator gives you the dose for your tank.

What each source counts as cycled

They all run the same test, but the cutoffs differ a little. Dr. Tim’s even uses two. Its printed recipe calls the tank cycled once ammonia and nitrite are both under 0.5 ppm 2. Its online quick guide treats that as “close to being cycled” and asks for one more round. Add another 2 ppm once both are under 0.2, and the tank is cycled when both are still under 0.2 ppm the next day 3.

If you want one rule, use the strict one, the same one as in the nitrogen cycle guide: dose, wait 24 hours, and look for zero on both readings. Passing twice in a row costs one more day and settles it.

Readings that fool people

A good number can still mislead you if you read it at the wrong time.

Zero ammonia with nothing going in. Some people stop dosing, wait until everything reads zero and call the tank cycled. That tank is only cycled for tiny amounts of ammonia. Add too many fish or too much food and you’ll get an ammonia spike 6. Zero means something only if the tank was fed ammonia the day before.

No nitrate. You’d expect it to show up. But nitrifying bacteria can take up nitrate themselves, and so can plants, so a cycled tank can read zero 5. Nitrate doesn’t have to be there for the tank to count as cycled 4.

Ammonia gone, nitrite still high. That’s the halfway point. The ammonia eaters are working and the nitrite eaters haven’t caught up. Nitrite has to reach zero before the tank counts as cycled 4. How long that takes is covered in how long to cycle a fish tank.

Ammonia still there after 24 hours. Not done. One reported cycle still had ammonia left 24 hours after dosing at nine weeks (63 days), and that’s not unusual 5. If you’re stuck, check pH first. The fishless cycle guide covers stalls. Ammonia in a tank that was already cycled is a different problem, covered in high ammonia in a fish tank.

Cloudy white water. Not a sign the bacteria have arrived. Nitrifying bacteria can’t grow fast enough to cloud the water 1.

Check your units

Dr. Tim’s numbers are ammonia and nitrite measured as nitrogen (NH3-N and NO2-N) 2. Some kits report the whole molecule instead, which reads higher for the same water. At zero it makes no difference. Around the 0.2 to 0.5 cutoffs it can. The nitrogen cycle guide has the conversion table.

Planted tanks

Plants soak up ammonia too, which muddies the test. A fair rule for a planted tank is to wait until the new plants put out new leaves and shoots and ammonia falls below 0.25 ppm 24 hours after a dose 7. A stricter version is to test first thing in the morning after dosing 1 to 2 ppm at lights-out the night before. Our reading is that this keeps the plants, which do most of their ammonia uptake under the lights, from taking the credit. Even the keeper who suggests it calls the cutoff arbitrary 7.

It passed. Now what?

Do a partial water change before any fish go in 3. If nitrate built up during the cycle, make it a 50% change above 80 ppm and 80% at 160 ppm 4. Already have fish in a tank that isn’t cycled? See cycling a tank with fish in it. Lost fish in a new tank? See what usually killed them.

Then stock slowly. On Dr. Tim’s chart the first fish go in on day 9, with a 25% water change on day 14 and every two weeks after that 2. Bacteria take time to grow back to full strength, so feeding should go up gradually 8. That advice comes from fish farms restarting a system. We think the same caution fits a new home tank, so add a few fish at a time and feed lightly for the first weeks. How many is a few? Our guide to stocking a 10 or 20 gallon tank gives school sizes and example stocklists. A brown, dusty film on the glass is common at this stage, too. Here’s what it is and when it fades.

Fish not arriving yet?

The bacteria live on the ammonia you feed them. Stop feeding and some of them die off 8. If there’ll be days or weeks before the fish come home, keep adding ammonia to keep the filter going, as fish farms do between batches 8. Run the 24-hour test again the day before you buy fish.

FAQ

How do I know if my tank is cycled? Dose a full 2 ppm of ammonia and test 24 hours later. If ammonia and nitrite both read zero, it’s cycled 1. Some sources accept up to 0.2 to 0.25 ppm 3^,4.

Does nitrate have to show up? No. Nitrifying bacteria and plants can take nitrate up, so a cycled tank can read zero 5. Nitrate doesn’t have to be there for the tank to count as cycled 4.

My ammonia and nitrite read zero but I haven’t dosed in days. Am I done? Not yet. Zero only counts if the tank was fed ammonia the day before. Otherwise it’s cycled only for tiny amounts of ammonia, and too many fish or too much food will cause a spike 6.

Does cloudy white water mean the bacteria have arrived? No. Nitrifying bacteria can’t grow fast enough to cloud the water 1.

There’s still ammonia after 24 hours. What now? It’s not done. One reported cycle still had ammonia left 24 hours after dosing at nine weeks (63 days), and that’s not unusual 5. Check pH first. The fishless cycle guide covers stalls.

What do I do once it passes? Do a partial water change before any fish go in 3, then add fish a few at a time and feed lightly at first.

Disclosure: Dr. Tim Hovanec, whose recipe this guide uses, was Chief Science Officer of Aquaria Inc. (Marineland’s parent company) and is President of DrTim’s Aquatics, which sells the products the recipe calls for 9. We have no relationship with the company.