The Hows and Whys of Fish Tank Cycling


You might have just purchased a fish tank and accouterments, including substrates, filtering systems, decorations, and more. If so, congratulations! We know you can’t wait to add the underwater species to their new habitat.

One final step remains before introducing finned friends into their  tank and your life: Cycling your new tank. Fish tank cycling takes time. But it’s also essential to ensure a healthy home for your fish.

The Nitrogen Cycle

To understand tank cycling, knowing about your tank’s nitrogen cycle is essential. That nitrogen cycle turns a potentially deadly environment for your fish into one where they thrive.

The nitrogen cycle starts when you feed your pet. After they eat, fish produce waste in the form of ammonia. Without help, that ammonia can build up in the tank, which can spell trouble for the fish.

The good news? Nitrifying bacteria to the rescue! These microorganisms reside on the tank’s interior surfaces, as well as on the water filters and amid the substrate. They dine on ammonia waste, converting it into nitRITES. The not-so-good-news? While this eliminates the ammonia, high nitrite levels can also cause problems for your fish.

Fortunately, a second strain of nitrifying bacteria is in place to “eat” those nitrites, creating nitRATES. The nitrates are benign to your fish. So when all operates appropriately in your tank, your fish aren’t subject to those highly toxic ammonia and nitrites.

A well-maintained, established tank has enough of the right kinds of bacteria to help keep a healthy underwater ecosystem. But this isn’t the case for brand-new tanks, which have little to no bacteria.

Here’s the good news: Tank cycling allows you to invite that bacteria into your fish’s environment.

The Tank Cycling Process

There are two types of tank cycling: fish-in-cycle and fishless cycle. The fishless cycle is considered by many to be more humane, as it’s accomplished without fish in the tank. Failure to monitor ammonia and nitrite levels could be dangerous to your fish. But when done correctly, the fish-in-cycle method can be accomplished safely.

To successfully cycle your tank (either empty or with fish), you’ll need ammonia, nitrite, and nitrate testing kits in addition to the other items for your tank.

Fishless Cycle: For Your Empty Tank

Follow these steps for a successful fishless tank cycle:

  1. Set up your tank, putting pumps, heater, quality water filters, measuring devices, and substrates into place.
  2. Fill the tank with non-chlorinated water.
  3. Develop ammonia with either of these daily methods:
    • Sprinkle fish food and allow it to decay
    • Add pure, unscented ammonia (with no additives) to the water
  4. Continue adding the food or ammonia to the water daily and monitor the levels. The ammonia levels should spike first, followed by nitrite levels.
  5. Keep an eye on both ammonia and nitrate levels, which should decline over a four-to-six-week period. Once these levels reach 0 ppm, nitrates have taken over, and the water is safe for your fish.
  6. Introduce the fish to the tank one or two at a time. Be sure to check ammonia and nitrite levels. If they remain at 0 ppm, you can add more fish to the tank. Or stop by Pets on Broadway, and we’ll gladly test your water at no charge.

Fish-in-cycle: When Fish Are at Home

There are two ways to cycle your tank with fish: Using an ammonia detoxifier and conditioner (like Seachem Prime) or through multiple water changes. In both methods, you’ll want to follow steps #1 and #2 above – prepare your tank with filters and substrate and fill it with non-chlorinated water. Then, add the fish.

If you decide to perform a fish-in-cycle with Seachem Prime:

  • Feed your fish lightly (you want to ensure less waste, meaning less ammonia).
  • Use your ammonia kit to test every 24 hours.
  • When ammonia appears and increases, dose with Seachem Prime (following the instructions on the bottle).
  • When nitrites appear, continue dosing with Seachem Prime – and test frequently.
  • When nitrates appear, continue testing for all three (ammonia and nitrates) and dosing with Seachem Prime.

If you decide to perform a fish-in-cycle process with water changes (and no extra chemicals), you’ll still want to feed your fish lightly and test frequently for a rise in ammonia. Your goal is to keep ammonia levels at 0.15 parts per million (ppm) in the tank. Once ammonia levels reach 0.5 ppm, remove 50% of the water from your aquarium and replace it with dechlorinated fresh water.

Continue this process until nitrites appear. Then, monitor ammonia and nitrite levels daily, changing water as both increase. Continue these actions until nitrite and ammonia levels decline and nitrates appear. Once the ammonia and nitrite levels reach zero, your tank is cycled. The bacteria are doing their job, and the environment is safe for your fish.

The fish-in-cycle process can take longer to achieve. Additionally, you could lose a fish or two with this method, especially if ammonia and nitrite levels are high and your fish are sensitive to their presence.

If you have any questions about the quality of your water, be sure to stop by Pets on Broadway for a free water test.


Keeping Your Fish Safe – and Healthy

Though tank cycling is time-consuming, it’s a necessary step so you can keep your fish healthy. Tank cycling adds bacteria to your underwater environment to develop and maintain a flourishing nitrogen cycle. Whether you use the fishless cycle or fish-in-cycle methods, add fish sparingly to your tank - and monitor those ammonia and nitrite levels.

Did you know that we remodeled our Fish Department at the end of 2023? We have added new styling, additional tanks, and an entire new layout. Stop by and see the new and improved fish room today!