Is my tank cycled? How to tell for sure

A tank is cycled when its bacteria turn a full dose of ammonia into nitrate within a day. Here is how to check that with an ordinary test kit, and which signs can fool you.

What “cycled” actually means

Fish release ammonia through their gills and waste, and leftover food adds more. Ammonia harms fish even in small amounts. In a cycled tank two groups of bacteria, living mostly in the filter, deal with it: the first turns ammonia into nitrite, the second turns nitrite into nitrate. Nitrate is far milder, and water changes remove it.
So “cycled” means both groups are big enough to keep up. Clear water, a running filter or a number of days can’t tell you that. Only water tests can.

The 24-hour test

The most reliable check takes one day:
  • 1.
    Test ammonia, nitrite and nitrate, so you know where you start.
  • 2.
    Add ammonia until the tank reads about 2 ppm.
  • 3.
    Wait 24 hours without adding anything.
  • 4.
    Test again.
The test passes when ammonia is 0, nitrite is 0 and nitrate is present. That means the bacteria processed the whole dose within a day and carried it all the way to nitrate.
One pass can be luck, so repeat it the next day. Two passes in a row is a solid yes. CycleCheck keeps score for you: tap “I added ammonia” after dosing, log the next test, and it counts the passes.

What the readings mean along the way

  • Ammonia high, nitrite 0: the cycle has barely begun. The first bacteria are still growing.
  • Nitrite appears: the first group works. The numbers look worse, but this is progress.
  • Nitrite falls while nitrate climbs: the second group is catching up. You are close.
  • Ammonia and nitrite both 0 a day after a full dose, twice in a row: cycled.

Signs that fool beginners

  • Ammonia dropped. Live plants take up ammonia too, and a misread test can show a drop. If nitrite never appeared and nitrate isn’t rising, the bacteria haven’t proven anything yet.
  • The water is crystal clear. Clarity says nothing about ammonia or nitrite. A milky bacterial bloom in a new tank is common and harmless too.
  • It has been 4 weeks. Time is only a rough guide. Low pH, cold water or tap water without conditioner can stall a cycle for weeks.
  • The bottle said “instant”. Bottled bacteria can speed things up, but the 24-hour test still decides.

Before you add fish

A finished cycle leaves a lot of nitrate behind. Do a big water change, using water conditioner, until nitrate is 30 ppm or lower, and then add fish. Keep testing during the first weeks with fish, and change 30⁠–⁠50% of the water if ammonia or nitrite ever reaches 0.25 ppm.

Getting honest readings

  • Liquid test kits are usually more accurate than strips.
  • Shake nitrate bottle no. 2 hard before use; it settles and gives readings that are too low.
  • Compare the colours in daylight, holding the tube against the white part of the card.
  • Test your tap water once. It can contain some ammonia (from chloramine) or nitrate.

Check your own tank

Log your test results and CycleCheck tells you which stage your tank is at, what to do today, and when the 24-hour test has passed twice in a row.
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