Cycling builds the bacteria your tank needs to keep fish alive

Cycling an aquarium means running the tank empty (or with only plants and decorations) for two to four weeks while beneficial bacteria grow on the filter media, substrate, and surfaces. These bacteria convert ammonia from fish waste into nitrite, then into nitrate — a less toxic form that water changes remove. Without this bacterial colony, ammonia and nitrite spike to levels that poison or kill fish within days.

You do not need special products or expensive equipment. You need time, a way to introduce ammonia into the water, and a test kit to track when the cycle is complete. Most aquariums cycle in 30 to 45 days, though some take longer depending on temperature and the method you choose.

Key Takeaways

  • Cycling takes 30 to 45 days and requires ammonia to feed the bacteria that will later handle fish waste.
  • The fishless method (adding ammonia directly) is faster and safer than the fish-in method, which stresses or harms the fish.
  • You need an aquarium test kit that measures ammonia, nitrite, and nitrate — not just pH or temperature.
  • The cycle is complete when ammonia and nitrite both drop to zero within 24 hours after you add ammonia.
  • Temperature matters: warmer water (75–80°F) cycles faster than cold water, because bacteria grow more quickly.

The fishless cycling method: adding ammonia directly

The fishless method is the most reliable way to cycle. You add a source of ammonia to the empty tank, let bacteria colonize, and test the water every few days until the cycle completes. This method takes 30 to 45 days and does not expose fish to toxic water.

The most common ammonia source is pure ammonia (also called ammonium hydroxide), sold at hardware stores and online as a cleaning product. Buy the kind with no dyes, perfumes, or surfactants — the label should say "pure" or "100% ammonia." Add enough to reach 2 to 4 parts per million (ppm) of ammonia in your tank. A 20-gallon tank needs roughly 1 teaspoon of pure ammonia; a 55-gallon tank needs about 2.5 teaspoons. Test the water after adding it to confirm the ammonia level.

An alternative is ammonium chloride, sold as a dry powder at aquarium stores under brand names like Fishless Fuel or Fritz Turbo Start. Follow the package instructions for your tank size. Some people also use a dead shrimp or fish food, but these methods are slower and less predictable because the ammonia release depends on decomposition speed.

Once you have added ammonia, do nothing else for 24 hours. Then test the water. Over the next two to four weeks, you will see ammonia drop, nitrite rise and then drop, and nitrate rise. When ammonia and nitrite both read zero and nitrate is above 5 ppm, the cycle is complete.

The fish-in cycling method: if you must add fish when ready

The fish-in method means adding hardy fish to the tank while bacteria are still growing. This stresses the fish and can cause illness or death, so it is not recommended. However, if you have already added fish, you can still cycle the tank — it will just take longer and require more frequent water changes to keep ammonia and nitrite from reaching dangerous levels.

If you are cycling with fish in the tank, test the water every other day. When ammonia or nitrite rises above 1 ppm, do a 25 to 50 percent water change. Keep doing this until both ammonia and nitrite stay at zero for several days in a row. This usually takes 6 to 8 weeks because the fish produce ammonia, but the bacteria are not yet established enough to process it quickly.

Use only hardy species for fish-in cycling — plecos, danios, and some catfish tolerate poor water quality better than tetras, gouramis, or bettas. Even hardy fish may show signs of stress: clamped fins, gasping at the surface, or refusing food. If this happens, do a larger water change (50 percent) when ready.

Testing the water: what to measure and when

You need a test kit that measures ammonia, nitrite, and nitrate. Liquid test kits (like the API Master Test Kit) are more accurate than test strips, though strips are faster. Do not rely on pH or temperature tests alone — they do not tell you whether the cycle is progressing.

Test the water on day one (after adding ammonia), then every two to three days for the first two weeks. After that, test once a week. Write down the numbers so you can see the pattern. A normal cycle looks like this: ammonia drops from 2–4 ppm to 0 over 2–3 weeks, nitrite rises to 2–5 ppm around week two, then drops to 0 by week four or five, and nitrate climbs steadily throughout.

If ammonia or nitrite stays high after four weeks, the tank may be too cold, the filter may not be running, or there may not be enough surface area for bacteria to grow. Check that the filter is on 24 hours a day, raise the temperature to 75–80°F if possible, and add more decorations or a sponge filter to increase bacterial surface area.

Temperature and filter setup: what speeds up cycling

Bacteria grow faster in warm water. A tank at 75–80°F cycles in 30 to 45 days; a tank at 65–70°F may take 60 to 90 days. If you have a heater, use it during the cycling period. You can lower the temperature back to normal once the cycle is complete and fish are in the tank.

The filter must run 24 hours a day, every day. Bacteria live on the filter media (sponge, ceramic, or biological media), so the filter is where most of the colony grows. If you turn the filter off, the bacteria will start to die within a few hours. Do not clean the filter media with tap water during cycling — chlorine kills bacteria. If the media gets clogged, rinse it gently in old aquarium water that you have removed during a water change.

A larger filter cycles faster than a small one because it has more surface area for bacteria to colonize. If your filter seems undersized, you can add a second filter or a sponge filter during cycling. Once the cycle is complete, you can remove the extra filter or keep it running for backup.

What to avoid during cycling

Do not do large water changes unless ammonia or nitrite reaches dangerous levels (above 4 ppm). Water changes remove the ammonia that bacteria need to eat, which slows the cycle. Small changes (10 to 15 percent) are fine if needed, but the goal is to let ammonia and nitrite build up so bacteria have food.

Do not add fish food, plants, or decorations that will rot. Dead material produces ammonia, which can spike the levels unpredictably. Live plants are fine — they actually help by consuming some ammonia and nitrate — but remove any uneaten food or dead leaves.

Do not use tap water conditioner that removes ammonia (some brands do this). Use only a conditioner that removes chlorine and chloramine. Check the label to be sure.

When the cycle is complete and how to add fish

The cycle is done when ammonia reads 0 ppm and nitrite reads 0 ppm 24 hours after you add ammonia. Nitrate should be above 5 ppm. At this point, the bacteria are established and ready to handle fish waste.

Do a 50 percent water change before adding fish. This removes excess nitrate and brings ammonia back down to zero. Then add fish slowly — do not stock the entire tank on day one. Add a small group (one-third of your planned fish) and wait a week. Test the water daily. If ammonia and nitrite stay at zero, add another group. This gradual stocking lets the bacteria population grow to match the fish load.

After fish are in the tank, test the water once a week for the first month, then once every two weeks. If ammonia or nitrite rises above 0.5 ppm, do a 25 percent water change and check that the filter is clean and running. Once the tank is stable, you can test less often — many experienced aquarists test once a month.

Frequently Asked Questions

Can I speed up cycling with bacteria products?

Bottled bacteria products (like Seachem Stability or Fritz Turbo Start) may help, but they are not necessary. Some work better than others, and results vary. If you use one, follow the package instructions and still test the water regularly — do not assume the cycle is complete just because you added the product.

What if my ammonia or nitrite gets stuck at a high level?

This usually means the tank is too cold, the filter is not running, or there is not enough surface area for bacteria. Check that the filter is on and the heater is set to 75–80°F. If the filter media is clogged, rinse it gently in old tank water. If it is still stuck after a week, do a 50 percent water change and add more filter media or a sponge filter.

Do I need to do water changes during cycling?

No, unless ammonia or nitrite rises above 4 ppm. Water changes remove the ammonia that bacteria need to grow, so they slow the cycle. If levels get dangerously high, a 25 to 50 percent change is safe and necessary.

Can I cycle a tank with plants instead of ammonia?

Plants alone do not produce enough ammonia to cycle a tank quickly. You still need an ammonia source. Live plants help by consuming some ammonia and nitrate, which can speed things up slightly, but they are not a replacement for adding ammonia or fish.

How do I know if my cycle has crashed?

A crash means the bacteria colony has died, usually because the tank was too cold, the filter was off for too long, or the water was treated with something toxic. If ammonia stays high and nitrite drops to zero (instead of rising first), the cycle may have crashed. Start over: raise the temperature, confirm the filter is running, and add ammonia again.