Ammonia in a new aquarium starts the nitrogen cycle your fish need to survive

When you set up a new aquarium, the water contains no beneficial bacteria yet. Fish produce ammonia as waste, and those bacteria convert it into less toxic compounds. Until the bacteria colony grows large enough, ammonia builds up and poisons the water. Adding ammonia yourself jumpstarts this process before you add fish — a method called fishless cycling.

You add pure ammonia (not household cleaner) to create a food source for bacteria. Over two to four weeks, the ammonia drops as bacteria multiply, then nitrite appears and drops, then nitrate appears and stays. Once you see this pattern, the tank is ready for fish. This prevents the ammonia spikes that kill fish in newly stocked tanks.

Key Takeaways

  • Pure ammonia (ammonium chloride solution with no additives) is the only safe form to add; household cleaners contain surfactants and other chemicals that harm bacteria and fish.
  • You dose ammonia to reach 2 to 4 parts per million (ppm) in the tank, then test daily with a liquid test kit to track when it drops and nitrite appears.
  • The cycle takes two to four weeks and is complete when ammonia and nitrite both read zero and nitrate is present, meaning bacteria are established.
  • Once cycling is done, never add ammonia again during normal tank operation — fish waste provides all the ammonia the bacteria need.

What kind of ammonia to buy and where to find it

You need pure ammonia solution, usually sold as ammonium chloride or ammonium hydroxide with no additives. Look for products labeled "fishless cycle ammonia" or "pure ammonia" at aquarium stores, online retailers, or sometimes at chemistry supply shops. The bottle should list only ammonia and water as ingredients — no surfactants, dyes, or perfumes.

Common brands include Flourish Nitrogen (made by Seachem), Fritz Turbo Start, or generic ammonium chloride solutions. A small bottle lasts months because you use only a few drops per dose. Avoid household ammonia cleaners entirely; they contain detergents that kill the bacteria you are trying to grow and can harm fish later.

If you cannot find ammonia locally, online aquarium retailers stock it and ship quickly. The cost is usually between $8 and $15 per bottle, and one bottle is enough to cycle multiple tanks.

How to dose ammonia correctly

Your goal is to reach 2 to 4 parts per million (ppm) of ammonia in the tank. The exact amount depends on your tank size. A general starting point is 2 drops per gallon for a 10-gallon tank, but the only way to know if you hit the target is to test the water.

Add the ammonia, wait 24 hours, then test with a liquid test kit (not strips, which are less accurate). If the reading is below 2 ppm, add a bit more. If it is above 4 ppm, do a partial water change to dilute it. Once you hit 2 to 4 ppm, stop adding ammonia and test daily. The bacteria will consume the ammonia over the next few days.

When ammonia drops to zero, nitrite will appear. Add ammonia again to bring it back to 2 ppm, and repeat this process. Once both ammonia and nitrite read zero on the same day and nitrate is present, the cycle is complete.

Testing during the cycling process

A liquid test kit (also called a master test kit) measures ammonia, nitrite, and nitrate. Brands like API Master Test Kit are standard and cost $25 to $35. You will use it every day during cycling, so it pays for itself quickly. Test strips are cheaper but often give false readings, especially for ammonia.

Test at the same time each day and record the results. You are looking for this pattern: ammonia drops to zero, then nitrite appears and rises, then nitrite drops to zero while nitrate rises. This cycle usually takes two to four weeks. Some people speed it up by adding a bacterial starter culture (like Seachem Stability or Fritz Turbo Start), which introduces bacteria directly and can cut the time to one to two weeks.

Keep the tank at room temperature (68 to 78 degrees Fahrenheit) and do not add fish or food during this time. The ammonia you are adding is the only food the bacteria need.

What happens after the cycle is complete

Once ammonia and nitrite both read zero and nitrate is measurable, the tank is cycled. The beneficial bacteria are now established in the filter media and substrate. You can now add fish gradually — start with a small number and wait a few days before adding more, so the bacteria population can adjust to the new waste load.

From this point forward, never add ammonia again. Fish waste produces all the ammonia the bacteria need. Your job is to do partial water changes weekly to remove the nitrate that builds up (bacteria convert ammonia to nitrate, but nothing removes nitrate except water changes and plants).

If you ever see ammonia or nitrite spike after fish are in the tank, it usually means the filter is clogged, the tank is overstocked, or you are overfeeding. Clean the filter gently in old tank water (not tap water, which kills bacteria) and reduce feeding.

Fishless cycling versus fish-in cycling

Fishless cycling (using ammonia) is safer than adding fish to an uncycled tank. Fish produce ammonia naturally, but during the first two weeks, ammonia and nitrite can spike high enough to damage their gills and organs. Some fish survive this, but many do not, and survivors are stressed and prone to disease.

If you have already added fish to an uncycled tank, do not panic. Do 25 percent water changes every other day to keep ammonia and nitrite below 1 ppm, and the cycle will still happen — it just takes longer and stresses the fish. Once ammonia and nitrite both read zero, you can return to normal maintenance (weekly 25 percent changes).

Common mistakes to avoid

The most common mistake is adding too much ammonia at once. High ammonia (above 5 ppm) can actually slow bacterial growth. Stick to 2 to 4 ppm and test before adding more.

Another mistake is doing large water changes during cycling. If ammonia is high, a 25 percent water change is fine, but do not drain half the tank — you will dilute the ammonia and reset the bacteria's food supply, extending the cycle.

Do not use household ammonia, do not add fish food (it rots and clouds the water without feeding bacteria), and do not assume the cycle is done without testing. A tank that looks clear may still have dangerous ammonia or nitrite.

Frequently Asked Questions

Can I use household ammonia cleaner instead of pure ammonia?

No. Household cleaners contain surfactants and other chemicals that kill the bacteria you are trying to grow. They also leave residue that harms fish later. Buy pure ammonia solution made for aquariums or chemistry use only.

How long does fishless cycling usually take?

Two to four weeks is typical. Adding a bacterial starter culture can cut it to one to two weeks. Temperature matters too — warmer water (75 to 78 degrees) cycles faster than cooler water (68 to 72 degrees).

What if ammonia stays high after two weeks?

High ammonia usually means bacteria are not growing fast enough. Check that the filter is running, the temperature is in the right range (68 to 78 degrees), and you are not adding too much ammonia at once. If ammonia is above 5 ppm, do a 25 percent water change to bring it down to 2 to 3 ppm.

Do I need to add ammonia if I am using a filter from an established tank?

No. A filter from an established tank already contains bacteria, so the cycle happens much faster — usually within a few days. You can add fish sooner, but still test daily for the first week to make sure ammonia and nitrite stay at zero.

What should I do with the ammonia bottle after cycling is done?

Store it in a cool, dark place. A small bottle lasts months or years because you use only drops at a time. Keep it out of reach of children and pets, and never pour it down the drain in large amounts — dilute it with water first.