More fish die in the first weeks of a new aquarium than from disease or bad food. The water looks perfectly clean all the while: what kills the fish has neither colour nor smell.

What happens in a new tank

Fish release ammonia — through the gills and in their waste. Decaying food and any organic matter produce it too. Ammonia is toxic: it burns the gills, and the fish suffocates in water full of oxygen.

Ammonia is removed by bacteria. They do not appear instantly — they need weeks to colonise the filter media and the substrate. They work in two steps:

  1. One group turns ammonia into nitrite. Nitrite is no less toxic: it blocks the transport of oxygen by the blood.
  2. Another turns nitrite into nitrate. Nitrate in reasonable amounts means little to the fish, and it is removed by water changes.

That chain is the nitrogen cycle. Until it is running, an aquarium is not a habitat but a sealed container with an accumulating poison.

An important detail about ammonia

Ammonia exists in water in two forms, and only one of them is truly toxic. The ratio depends on temperature and, above all, on pH: the warmer and the more alkaline the water, the greater the share of ammonia in its dangerous form.

Hence a consequence: the same test reading is more dangerous in hard alkaline water than in soft acidic water. And hence the rule: do not raise the pH while ammonia is above zero. Raising it turns the relatively harmless form into the caustic one.

Fishless cycling

The bacteria need feeding, that is, a source of ammonia. Fish are not required for that, and it is better without them: a "fish-in cycle" means a living creature spends weeks in caustic water.

The procedure is simple:

  1. Fill the tank and run the filter, heater and aeration around the clock. The temperature has to be at working level: in cold water the bacteria grow twice as slowly.
  2. Add a source of ammonia: a pinch of food every couple of days, or additive-free ammonia solution.
  3. Test ammonia and nitrite every two or three days.
  4. First ammonia rises, then it falls and nitrite shoots up, then nitrite falls too and nitrate begins to rise.

Cycling is finished when ammonia and nitrite stay at zero and nitrate is rising. That usually takes three to six weeks. Neither cloudiness, nor clarity, nor "a month has passed" is a sign — only tests are.

What speeds it up

  • Media from a running filter is the most reliable route: an established colony moves with it. A squeezed sponge or a handful of ceramic media cuts cycling down to a few days.
  • Live plants take up ammonium themselves and bring bacteria along.
  • Bottled bacteria vary: some products really do shorten the process, others do nothing. Testing is required either way.
  • Warmth and aeration. These bacteria breathe oxygen; in still water the colony grows slowly.

What does not speed it up: water changes "for freshness", a full water change, antibiotics and any medication. Medicines in a new tank hit the very colony you are growing.

After cycling

  • Stock in batches, not all at once: the colony grows to match the load, and ten new fish in one go cause an ammonia spike even in an established tank.
  • Wash the filter in aquarium water drawn off during a water change, never under the tap: chlorine from the mains kills the bacteria. Squeeze the media rather than scrubbing it clean, and replace it in parts, never entirely.
  • Treat tap water with a conditioner. Chlorine and chloramines burn gills and undermine the biofilter.
  • Do not wash the substrate through — the second largest colony lives in it.

Cloudiness in the first week

A milky haze a few days after filling is a bloom of bacteria in the water column. It is a normal part of cycling and clears by itself within days. Changing the water, cleaning the filter and "restarting" the tank at that moment only draws the process out.

If ammonia or nitrite spikes with fish in the tank

  1. Stop feeding for a day or two. Hunger will not harm the fish; ammonia will.
  2. Change 30–50 % of the water, matched in temperature and treated with conditioner, until the test shows safe values.
  3. Increase aeration.
  4. Do not raise the pH and do not raise the temperature.
  5. Add no new fish and treat nothing until the readings normalise.

More on pH and hardness in the article Water chemistry. On the signs of illness — and on what often turns out to be poisoning rather than infection — see Fish diseases.