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Cycling the Tank

Cycling the tank is a crucial step. The goal is to establish colonies of beneficial bacteria that will break down uneaten food and fish waste and convert them into non-toxic compounds. This is known as the nitrogen cycle, which is why it is called "cycling". Without the bacteria anything that decays in the tank becomes ammonia, which is very toxic and can kill everything in the tank.

We have to end up with two main types of bacteria: one type that turns ammonia into nitrite, which is still toxic and another that turns nitrite into nitrate, which is harmless in reasonable amounts.

To accomplish this, we're going to start by adding fish food to the tank, letting it decay and measuring the level of ammonia in the water. Once there is a good amount of ammonia, we're going to add a bottle of bacteria. As the bacteria get established and reproduce, we should notice the first type start to consume the ammonia and generate nitrite. Then, the second type should start to increase in numbers and will consume the nitrite and turn it into nitrate. The cycle is done when there is no ammonia or nitrite in the water and some amount of nitrate. At that point, we can put living things in the tank.


To seed our tank with bacteria we're going to use Dr. Tim's One and Only Live Nitrifying Bacteria. Visit the product page for a ton of useful information. We're going to buy enough to treat twice the volume of our tank since we will likely add bacteria twice. For this tank, I bought a 4 oz bottle which treats 60 gallons. This stuff is pricey, but well worth it. Make sure you get the saltwater version and that you keep it in the fridge until it is needed.


Above all, cycling the tank requires a great deal of patience. It could take several weeks and it's hard to resist the temptation of adding living things to the tank. So, be prepared to wait.

There are many ways to cycle a tank and I have done it differently for every one I've cycled. This time, I'm trying something a little different and it may or may not work. It could be quick, or could end up taking a long time. But since we're going to be very patient, we'll get through it.

There are many products that claim to help accelerate the cycle and some folks recommend a quick cycle by adding bacteria and fish at the same time. We won't be doing that, but it is possible.


We're going to establish some rules that are specific to our cycling process.

  • If you already have lights, leave them off for the duration of the cycle. We won't actually need them until we add corals since bacteria and fish are fine without them, but algae and other pests love them.

  • If you have a protein skimmer, leave it off. A skimmer can also slow down the cycle at the start since it removes decaying matter.

  • If you have a UV sterilizer, leave it off. Although a UV sterilizer targets organisms that are in the water column and should not harm our rock coating bacteria, it's best to leave it off for now.

  • Don't do any water changes. They are not necessary while we cycle and can slow things down since they remove some of the compounds that the bacteria need to thrive and reproduce.

  • No living things. Don't add any living things to the tank while it is being cycled, they can die a painful death.


Before we get going, let's make sure everything is in order:

  • The tank is full of good saltwater made from zero TDS freshwater at or close to 1.025 sg.

  • The return pump is running and our mechanical filtration (the filter sock and fiber balls) is working properly.

  • The heater is working well and keeping the water consistently near 78°F.

  • The water is not cloudy.

  • The ATO system is running and maintaining the water level and salinity stable. The reservoir is full and you're checking it every few days.

  • You've tested pH and it is between 7.7 and 8.4.

  • You've tested ammonia, nitrite and nitrate and they are all zero.

  • You have a bottle of Dr. Tim's One and Only Live Nitrifying Bacteria.

  • You've armed yourself with patience and are ready to embark on the cycling adventure.

If any of the above are not as they should be, take action now to correct them before we start. Otherwise, let's move to day one of the cycle.