Cycling an aquaponics system without fish

Fishless cycling uses bottled ammonia to establish the nitrogen cycle before adding fish. It takes 3-6 weeks, protects the fish from ammonia exposure, and costs about $10.

Cycling is the process of establishing colonies of nitrifying bacteria in your system before adding fish. These bacteria convert toxic ammonia (produced by fish waste) into nitrite, then into nitrate, which plants absorb. Without an established colony, ammonia from the first fish accumulates to toxic levels within days, stressing or killing the fish before the bacteria can multiply to handle the load.

Fishless cycling solves this by providing ammonia from a bottle instead of from fish. The bacteria don't care where the ammonia comes from. They colonize the biofilter media, multiply to handle the ammonia input, and by the time you add fish, the system can process waste from day one.

What you need

Ammonia source: Pure ammonium chloride solution or household ammonia (must be pure, no surfactants, fragrances, or colorants). Check the label: the only ingredients should be ammonia and water. If shaking the bottle produces lasting foam, it contains soap and can't be used.

Ammonium chloride specifically formulated for aquarium and aquaponics cycling is available from aquarium suppliers for about $8-12. This is the safest option because the concentration is labeled and consistent.

Test kits: You need ammonia, nitrite, and nitrate liquid test kits. The API Freshwater Master Test Kit ($25-30) covers all three plus pH. You'll be testing every day or every other day during cycling, so strips aren't accurate enough.

A running system: The pump needs to be circulating water through the biofilter (grow bed media, sump, any filter media) the entire time. Bacteria colonize surfaces where oxygenated, ammonia-containing water flows over them. Without circulation, the bacteria only grow at the water surface.

The process

Day 1: add ammonia

Add ammonia to the system water until a test reads 2-4 parts per million. For pure ammonium chloride solutions, this typically requires about 1-3 mL per 100 liters of system water, but concentrations vary by product, so start small and test after each addition. Mix thoroughly by letting the pump circulate for 30 minutes before testing.

Temperature affects cycling speed. Nitrifying bacteria grow faster in warm water (25-30 C). If your system water is below 18 C, cycling will take significantly longer. A tank heater set to 25-28 C during the cycling period speeds things up.

Days 2-7: wait and test

Test ammonia every 1-2 days. Initially, the ammonia level stays where you dosed it because no bacteria are processing it yet. You may see a slight decline as ammonia is absorbed by surfaces and media.

Don't add more ammonia yet. Wait until the level drops measurably.

Days 7-14: ammonia starts declining, nitrite appears

Nitrosomonas bacteria (the first stage) begin converting ammonia to nitrite. You'll see ammonia levels falling and nitrite levels rising. This is the first sign that cycling is progressing.

When ammonia drops below 1 part per million, top it back up to 2-4 parts per million. The bacteria need a continuous food source to keep multiplying. If ammonia drops to zero and stays there for days, the colony may starve and shrink.

During this phase, nitrite levels can spike very high (10-20+ parts per million). This is normal and not harmful because there are no fish in the system.

Days 14-28: nitrite starts declining, nitrate appears

Nitrospira bacteria (the second stage) begin converting nitrite to nitrate. You'll see nitrite levels falling and nitrate levels rising on the test kit. Nitrate should be clearly detectable (5-40 parts per million or higher).

Continue topping ammonia back up to 2-4 parts per million whenever it drops below 1 part per million. The system needs to demonstrate that it can process a full ammonia addition to zero within 24 hours before it's ready for fish.

Days 21-42: cycle complete

The cycle is complete when both of these conditions are met simultaneously:

Ammonia drops from 2-4 parts per million to 0 parts per million within 24 hours of the addition. This confirms the first-stage bacteria are established in sufficient numbers.

Nitrite drops to 0 parts per million within 24 hours of the same addition. This confirms the second-stage bacteria are also established.

When both conditions hold for 3-5 consecutive daily doses, the system is cycled. You'll also have high nitrate readings (possibly 40-100+ parts per million) from all the ammonia that's been processed through the cycle. Do a large water change (50-80%) to bring nitrate down to a reasonable level (below 40 parts per million) before adding fish.

Timeline expectations

In warm water (25-30 C) with adequate aeration: 3-4 weeks is typical.

In cool water (18-22 C): 4-6 weeks.

In cold water (below 15 C): 6-8+ weeks. Consider using a heater during cycling even if the target species is a cold-water fish like trout. You can remove the heater after cycling.

Adding a bacterial starter product (bottled nitrifying bacteria from aquarium stores) can reduce cycling time by 1-2 weeks by seeding the system with live bacteria rather than waiting for them to colonize naturally from the environment. These products work, but results vary by brand and freshness. The bacteria are alive and die if the bottle has been on the shelf too long or stored in extreme temperatures.

Using media from an established aquarium or aquaponics system is the fastest way to seed the cycle. If you have a friend with a running system, a handful of their biofilter media placed in your system transfers an active bacterial colony directly. This can reduce cycling to 1-2 weeks.

Common mistakes

Adding fish too early. Testing once, seeing nitrate, and assuming the cycle is done. The cycle isn't complete until ammonia AND nitrite both reach zero within 24 hours of a full ammonia addition. Detectable nitrite means the second stage isn't ready. Adding fish at this point exposes them to nitrite toxicity.

Letting ammonia drop to zero for days between doses. The bacteria need continuous feeding. If ammonia stays at zero for more than 48 hours, the colony begins to shrink. Dose consistently.

Not running the pump. Bacteria colonize surfaces in the flow path. If the pump is off and water is static, bacteria only grow at the air-water interface, which is a tiny fraction of the available surface area. Run the pump 24/7 during cycling.

Using chlorinated water without treatment. Chlorine and chloramine kill nitrifying bacteria. If you're topping up the system during cycling, dechlorinate the water first.

The system sizing calculator can help you plan the system dimensions, and once cycled, the fish-to-plant ratio calculator helps determine how many fish to add.