Cycling a New Saltwater Aquarium With Fish

Using plastic bag to put tropical fish in tank

Dorling Kindersley / Dorling Kindersley / Getty Images

Imagine: You have picked out and purchased your new aquarium, set up and tested the equipment, started up the system, and if you are not cycling the tank with live rock, you are now ready to add some fish to get things going. You think to yourself, what am I going to put in it? You recall that you really liked that fish you saw in the store the other day, you know, the one that had all the bright colors. You remember it was so beautiful and very expensive, and at this point, you ask yourself, do I really want to spend $70.00 on a new fish?

Aside from being new to the hobby and having much yet to learn, for you to go out on a limb and spend this kind of money for a fish to put into a newly started aquarium is absurd, and don't let your local fish store salesperson talk you into it either. If you did your homework prior to starting your new aquarium, you know that the cycling period (the time it takes to reduce high levels of ammonia and nitrite that are toxic to fish, as well as all other animals) takes several weeks for establishing the biological filter in the aquarium. Considering the threats that loom ahead for any fish for the first 30 days or so, including your lack of experience, why would anyone want to put this kind of money into a fish to cycle a tank with?

To help you make good tank starter fish choices, here are some tips:

  • First of all, do not buy and introduce sensitive invertebrates or corals into the tank at any time during the cycling process.
  • When it comes to selecting starter fishes, as well as invertebrates for the future, plan ahead and prepare a "species want list," then learn as much as you can about each animal's characteristics, dietary requirements, feeding habits and captive care, BEFORE buying anything!
  • Using the measurement of approximately two inches of fish per 10 gallons of tank water rule of thumb can give you a general idea of how many fish to add, based on the size of your aquarium.
  • Buy inexpensive, durable, hardy fish that are known to do well during the tank cycling process. Some of the best fish to start an aquarium with are Damsels or Chromis fish, members of the Family Pomacentridae. Now we know some of you are going to say that these fish are so plain looking and can be very aggressive. Yes, they can be, but once the tank has cycled and allowed to run for a month or so, you can possibly trade them in at your local fish store or give them to someone else who may be starting a new aquarium. THEN you can step up to one or two of the more elegant beginner fishes.
  • Know what to look for when buying fish. The quality of the animals plays a vital role in your success with their care.

Did you know there are ways to speed up the nitrogen cycling process to help aid in the completion of the cycle? Commercial bacteria starter products are available that add the beneficial bacteria to your aquarium to help speed the cycling process.

During the cycling period, test your water chemistry frequently to measure ammonia, nitrite and nitrate levels. The most dangerous toxin that will naturally appear is ammonia, which is excreted by fish through their gills and is also created from fish waste and uneaten food. When the ammonia levels start getting a bit high, you can either perform a partial water change or treat the tank with an ammonia neutralizer to bring the levels back down into the safe zone. In order for the cycle to complete, there must always be a small amount of ammonia in the tank water, so do not continuously keep the ammonia level at zero. The small number of fish initially added are there to create the ammonia that the beneficial bacteria in the biofilter need to grow. But you don't want to risk their health by letting the ammonia or nitrite levels get too high. Do not add any more fish into the aquarium until the ammonia and nitrite levels are staying at zero, and the nitrate level is starting to rise.

Regardless of what method you use to cycle a new aquarium, the process is the same. Ammonia builds up until there are sufficient bacteria in the biological filter, gravel and on other surfaces growing to break ammonia down into nitrite. Just like ammonia, nitrite is toxic and harmful to marine animals even at lower levels. Nitrite will continue to rise to a high level until another species of bacteria grows in sufficient numbers to break down nitrite into nitrate.

These beneficial bacteria are living entities that require oxygen and food (a nitrogen source) to survive, and will grow on the surfaces of everything in the tank. The main biological activity occurs in the media of the filter because of the water flow through it bringing the bacteria the oxygenated water containing the nutrients they need. When nitrate readings begin to increase, that indicates the beneficial nitrifying bacteria are establishing themselves, which is what you have painstakingly been going through the cycling process to achieve.

At this point, it is safe to add a few more fish to the aquarium. Continue to monitor the ammonia, nitrite, and nitrate, as they will increase when you add new fish. When the ammonia and nitrite are down again after adding a few new fish, then you can add a few more fish or invertebrates until the aquarium occupancy reaches the maximum safe number of the fish you want. Keep the nitrate level down by performing partial water changes.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Aquarium Water Quality: Nitrogen Cycle. Florida Department of Agriculture and Consumer Services.

  2. Providing a Home for Fish. Merck Veterinary Manual.