Clear Water Isn’t Safe Water: How to Fishless Cycle an Aquarium in the UK

Oct 5

5 min read

Your new aquarium is set up. The filter is running, the gravel is clean, and the water looks crystal clear. It looks ready for fish — but appearances can be misleading.

Ammonia and nitrite are invisible. Both can build up in a new aquarium before its biological filter is established, putting fish at risk of poisoning.

Fishless cycling prepares your filter to process fish waste before any animals are introduced. It takes patience, an ammonia source and regular water testing. This guide explains the process for a freshwater aquarium.

What happens during cycling?

Fish release ammonia, mainly through their gills. Uneaten food, fish waste and decaying organic matter also contribute as they break down.

Beneficial microorganisms grow on filter media and other aquarium surfaces. They carry out two essential stages:

Ammonia becomes nitrite. Ammonia-oxidising microorganisms process the ammonia, producing nitrite, which is also harmful to fish.
Nitrite becomes nitrate. Nitrite-oxidising microorganisms convert nitrite into nitrate, which is generally less harmful but still needs managing.

Water changes and plant growth help control nitrate. An established filter does not remove the need for regular maintenance.

Fishless cycling supplies ammonia without using live fish, allowing this biological filtration to develop without exposing animals to the cycling process.

What you need

An aquarium ammonia source

Choose an ammonium chloride product intended for fishless cycling, with clear dosing instructions. Avoid household cleaning ammonia unless its complete ingredients and concentration have been verified. Perfumes, detergents and other additives have no place in an aquarium.

Water tests

You need to measure ammonia, nitrite, nitrate and pH. A liquid aquarium test kit is a practical option. Check what is included: some multiparameter strips do not test ammonia. Follow the shaking, timing and storage instructions carefully.

A way to prepare tap water

Use an aquarium conditioner suitable for chlorine and chloramine, following its label. If using an HMA unit, check its specific removal capabilities, flow limits and cartridge replacement schedule. The name “HMA” alone does not guarantee chloramine removal.

Optional: a bacterial starter

A suitable bacterial starter may shorten the process. Use the correct dose for your water volume and follow its storage and expiry instructions. Adding a bottle does not replace testing or guarantee a completion date.

Before you begin

Install the filter with its intended media and leave it running continuously. Provide good surface movement for oxygen exchange and use a heater if required for your planned aquarium.

Prepare the tap water before adding bacteria. Test and record your starting ammonia, nitrite, nitrate and pH so you know what is already present.

Base doses on the actual amount of water in the aquarium, allowing for space occupied by substrate and decorations.

Keep fish, shrimp, snails and other livestock out of the aquarium throughout ammonia dosing.

Step 1: Add a measured ammonia dose

Follow your aquarium ammonia product’s instructions for an initial dose of approximately 2 ppm. Products differ in strength, so never transfer a drops-per-litre dose from one brand to another.

Allow the water to circulate, then test as directed. Record how much you added. Do not keep adding ammonia simply to force an exact colour match: test kits and product instructions can use different reporting conventions.

If using a bacterial starter, add the amount specified on its label.

Step 2: Follow the readings

Test ammonia and nitrite every day or two and keep a simple record.

You may initially see little change. Later, ammonia should fall as nitrite appears. Nitrite can take longer to clear. There is no fixed day on which either change must happen.

Keep the filter running throughout. An unplanted aquarium can have its lights off during cycling; live plants still need an appropriate lighting period.

Use water tests to judge progress, not water clarity or the number of days since setup.

Step 3: Avoid excessive top-ups

An ammonia reading of zero does not mean the aquarium is ready if nitrite is still present.

For a conservative approach, wait until both ammonia and nitrite read zero before adding another measured dose. Do not automatically restore ammonia to 2 ppm every day while nitrite continues to accumulate.

If a result reaches the top of your test kit’s scale, pause dosing and make a partial water change with properly prepared water. Retest before proceeding. An off-scale result does not tell you how high the concentration actually is.

If progress stalls, check pH, temperature, filter flow and aeration. Low buffering capacity can allow pH to fall during cycling; investigate the cause before adding more ammonia or adjusting the water chemistry.

Step 4: Check the filter’s capacity

Once both ammonia and nitrite read zero, add a measured dose equivalent to approximately 2 ppm, following your ammonia product’s instructions.

Test again after 24 hours. Both ammonia and nitrite should read zero on your test kit. If either remains detectable, continue cycling before repeating the check.

Nitrate often rises during cycling, but a high nitrate reading alone does not prove the filter is ready. Tap water may already contain nitrate, while growing plants can absorb it.

Passing the ammonia-processing check demonstrates biological filtration capacity. The aquarium’s temperature, pH, hardness, size and stocking plan must also suit the fish you intend to keep.

Before adding your first fish

Test nitrate and change enough water to bring it to a level suitable for your chosen livestock. The amount needed depends on both the aquarium reading and the nitrate already present in your replacement water; a fixed 75% change will not suit every situation.

Prepare replacement water correctly and bring it close to the aquarium’s temperature. Keep the filter media wet during maintenance, and never wash it with soap or disinfectant.

After refilling, allow the water to circulate and recheck ammonia, nitrite, nitrate and temperature. Ammonia and nitrite should still read zero.

Introduce an appropriate initial stock, acclimatise the fish and feed modestly. Test ammonia and nitrite daily during the first week and after subsequent additions.

If you delay stocking, follow your cycling product’s maintenance instructions and repeat the capacity check before fish arrive. Stop dosing ammonia once livestock are present.

Need help with your cycle?

Join The Betta Box community to ask questions and share your progress. Include your aquarium size, temperature, test results, ammonia product and dosing history so other members have the information they need to help.

If you share test-tube photographs, take them against the kit’s colour chart in good, neutral light and include the readings you see in person.

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