How to Cycle a Saltwater Tank

How to Cycle a Saltwater Tank

Cycling a saltwater aquarium is essential to make it safe for livestock such as fish and corals.

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What Is An Aquarium Cycle?

The cycle of a marine aquarium refers to the nitrogen cycle aka ammonia cycle. It has to happen in order for aquarium to build the necessary, natural bacteria that protect your saltwater fish and to mature the water accordingly.

The bacteria which establishes during the cycle of the tank consume ammonia, nitrite, nitrates and other nutrients which may be harmful and toxic to your livestock. They literally feed off those nutrients and different types of bacteria form part of the cycle.

During a tank cycle an algae bloom will occur since the algae can and will successfully grow and flourish in the rise of nitrate. As your nitrates are consumed by the algae, the level of nitrates will drop and the algae will eventually die out.

How Does Cycling Work?


There are three major phases in a nitrogen cycle:

1. Ammonia (NH3/NH4)

Fist and foremost you need ammonia in your tank to start it’s cycle. The ammonia (NH3) also consists of inactive, non-toxic ammonium (NH4) which forms a much smaller part of the whole.

The ammonia is consumed by nitrosomonas and produce nitrite (NO2) as a result. The process is called nitrification. The nitrosomonas are naturally formed and there is really no way of preventing them from growing.

2. Nitrite (NO2)

Nitrite is already much less toxic to your fish compared to ammonia.

The nitrite that was produced in the nitrification process above will then be consumed by another type of bacteria called nitrobacter. In the process, they will produce nitrates (NO3) which is technically and arguably the end-product of the nitrogen cycle for the most part.

3. Nitrate (NO3)

Nitrates are again less toxic than both ammonia and nitrites.

Even though the nitrogen cycle is called what it is, the nitrates are the majority of the end-product even though it is believed that certain bacteria finishes the cycle by consuming nitrates and releasing nitrogen gas which evaporates from your aquarium directly into the atmosphere.

Apart from that, you’ll be stuck with some nitrates that you need to take care of. Reducing the levels of nitrates in your reef aquarium can be done in several ways such as regular water changes, carbon dosing and other debatable methods.

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In summary you’ll have an ammonia spike then a nitrite spike and lastly a nitrate spike. The cycle is now complete and your marine aquarium should be fish safe.

Testing During Cycling

As you cycle your saltwater aquarium, you want to test a few water parameters. It is very important!

  • Ammonia (NH3/NH4)
  • pH
  • Nitrite (NO2)
  • Nitrate (NO3)

The Seachem MultiTest Marine Basic provides you with tests for all these above mentioned parameters. Alternatively there are Salifert test kits for Ammonia, pH, Nitrite and Nitrate. I highly recommend that you get the Seachem Ammonia Alert badge so that you don’t have to constantly test for ammonia since it will suck/stick on the inside of the aquarium for constant results. The Seachem Alerts Combo Pack provides badges for both ammonia and pH. Awesome!

Ammonia is extremely toxic to your fish and can be fatal so be sure to check it constantly. Ammonia is also more toxic the higher the pH of the water is. If the ammonia rises, you can use Seachem Prime which will detoxify both ammonia, nitrite and nitrate in your aquarium. I always keep a bottle handy when cycling a saltwater aquarium just in case.

Ways to Cycle a Saltwater Aquarium

Before you start cycling, I will assume that you have your reef aquarium setup with the tank itself, saltwater at 1.026 salinity, a heater at roughly 26 degrees celcius and that you have your filtration/sump running with a return back into your tank. Then proceed.

There are several ways of successfully doing a marine tank cycle:

1. Cycle Saltwater Aquarium With Fish

Many reefers will consider the cycling of a reef tank with fish as cruel since you’re putting stress on the fish and may even cause the death of the fish. It is up to you if you want to use this method and how you morally feel about it.

To use this method, you’ll add a hardy fish to your newly setup saltwater tank. The goal of this cycling method is so that your fish will produce waste that decomposes into ammonia which then forms the nitrifying bacteria to start the cycle and feed of the ammonia produced and so forth.

Hardy fish include clown fish, damsel fish, cardinal fish, firefish, dartfish, wrasses, etc. They can withstand more stress compared to other species and should do fine in the fluctuation of water parameters.

2. Cycle Saltwater Aquarium Without Fish

To produce the ammonia in order to start your nitrogen cycle, you can add any type of protein to your aquarium that will decompose.

I recommend that you take a small piece of hake (2 by 2cm) or a piece of shrimp. Either should be raw, uncooked. You can buy these frozen or fresh at your local supermarket.

As it decomposes, it releases ammonia to start the cycle.

3. Cycle Saltwater Aquarium By Phantom Feeding

Phantom feeding is similar to the fishless cycle mentioned above except that you’ll be using fish pellets or flakes to cycle your saltwater aquarium.

While testing your ammonia levels every day or two, you’ll be adding a few pellets or flakes of marine fish food to the aquarium as if you’re feeding your fish. The fish food will decompose to release ammonia and start the cycle.

4. Cycle Saltwater Aquarium By Adding Ammonia

You can purchase pure ammonia and add that to your aquarium to start the cycle.

5. Cycle Saltwater Aquarium With Live Rock or Live Sand

Using live rock or live substrate is a great way of starting a saltwater aquarium.

The idea is that the live rock or live sand has organisms and bacteria on them that die off either during transport, setup or from ageing and then as a result release ammonia which is them consumed quite quickly by other, existing bacteria in/on the live rock and live sand.

6. Cycle Saltwater Aquarium With Established Media

Established media such as matrix or a filter sponge can be taken from an existing, established reef aquarium to start a cycle.

7. Cycle Saltwater Aquarium With Bacteria Additive Products

There are many products to start a marine aquarium nitrogen cycle. They basically provide dormant and animated bacteria in a bottle which you’ll add to your aquarium to start the cycle.

I recommend that you use Seachem Stability and add that to your saltwater aquarium to start and accelerate the cycle.

If you combine #1 and #7, you’ll have bacteria ready to protect your fish and consume the ammonia almost immediately as your fish produce waste. In my opinion that is the best combination. You’ll still see spikes and fluctuation in parameters as expected but your fish will be less stressed throughout the cycle as bacteria is already available to process nutrients.

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