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Week 17: How To Cycle a Saltwater Aquarium!

Unfortunately, just because we filled up our aquarium and setup all of our gear, doesn’t mean our saltwater aquarium is ready to be filled with fish and corals! Every new aquarium has to go through a “nitrogen cycle”. There are a few ways to do it, and its possible to speed it up, but you should expect to wait around 3 weeks on average before the cycle is complete. There’s also a few tips and things not to do that we will cover in this week!

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20 gallon saltwater aquarium from waterbox

Table of Contents

This Week's Video:

What Is Cycling?

Without getting too deep into biology or ecology, every natural aquatic system has a ton of bacteria that “eat” the toxic waste chemicals that bigger animals release. However, in a new aquarium, you first need to build up a strong population of these bacteria, otherwise your aquarium will have a really bad start. Maintaining the health of your bacteria in the long term is equally important, but that is a topic for another day. 

Usually a cycle takes about a month, but depending on if you use any products to help you, it may take more or less. It also helps to have a lot of surface area for bacteria to grow on, such as sand, porous rock, and other bio-media. The best way to know if your cycle is happening properly is to test your water’s ammonia, nitrate, and nitrite! 

The cycle is “complete” when the ammonia and nitrites are low, and nitrates should be somewhere from 0-40 ppm. 

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Three Ways to Cycle An Aquarium

Method One (Dead Shrimp Method)

The classic method is to get a dead shrimp piece (a large one) from the store, and to just place it somewhere within your aquarium. The shrimp acts as food, as well as having some ocean born bacteria likely in it. It is a good idea to put the shrimp in a net or a glass jar of some sort, as it will eventually start to fall apart. You also don’t need to leave it in forever, once the cycle is complete you can take it out right before you put in your fish. This is an easy method, but not super fast and it is smelly. 

Method Two (Live Fish)

This is a controversial method as it is somewhat cruel to the fish, but the general idea is to use hardy fish (that can tolerate the toxic chemical levels) and just feed them like you would normally. The fish will produce the food for the bacteria and it is usually faster than the shrimp method. However, this can often lead to the fish dying, and even if they survive, they usually aren’t the fish you want to keep, so you’ll have to go through the trouble of giving them away. 

Method Three (Fishless) 

Finally, the most “scientific” method is using ammonia with bacteria in a bottle. Thankfully many companies have products that come with directions that make it easy to do. This is probably the fastest and most humane method! 

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Things To Know During The Cycle

Don’t forget to test your water often to follow the cycle and to know when it is really complete! 

  • Don’t turn on your lights during the cycle
  • Top off your water but don’t do water changes
  • Don’t turn on your protein skimmer (if you have one)
  • Don’t turn on your UV Sterilizer (if you have one)
  • Don’t use filter socks

Even after your cycle is over, and you add your fish, there might be a second cycle that happens. As well, there is often an ugly stage that comes which looks like a lot of brown gunk we call diatoms. This is okay and just part of the process! 

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