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How to Cycle Your Aquarium

This article details the safest and most efficient aquarium cycling methods, and highlights some common cycling issues. If you’re not sure what cycling means, please read this article first: Introduction to the Nitrogen Cycle (Or, Why You Killed That Goldfish When You Were Little).

Seeded media method

This is the easiest, fastest and safest cycling method. “Seeded” filter media is any porous material that has been previously used in someone’s filter and still contains active nitrifying bacteria.

Step 1: Obtain the bacteria

If you have an established aquarium and are trying to set up a new one, simply transfer some of the biological media from that filter to your new one. Otherwise, you can ask a friend or your local fish store to give you some of their used filter media. (If you’re in the area, come see me and I will give you some of mine!) Once the media is in place, simply start your filter and let the bacteria do their work.

Step 2: Feed the bacteria

To keep the bacteria alive in your filter, you will need to provide them with a source of ammonia. Follow the ammonia or live-in methods from this point on. Note that the live-in method will be stressful for your animals. Using seeded media shortens the cycling process, but it could still take anywhere from a few days to a few weeks.

Ammonia method

This is the method I recommend if you have a brand new aquarium with no living animals in it. You can buy small amounts of pure ammonia from my shop, or find some in the cleaning section of most hardware stores. Some people prefer to let fish flakes or a raw shrimp decompose in their aquarium to create a source of ammonia, but this makes the process a lot slower and can introduce harmful pathogens to your tank. It’s also unreliable, as the amount of ammonia produced this way is extremely variable. Using pure ammonia allows you to control the exact concentration of ammonia that ends up in your tank.

Step 1: Obtain the bacteria

Aside from ammonia, you will need a source of bacteria. You could take your chances and wait for them to occur naturally in the water, but using bottled bacteria will speed things up considerably. You can obtain some from my shop or any pet store that sells fish. Any brand of bottled nitrifying bacteria will work. Pour them into the water and they will cling to whatever surface they find. The more porous surface they have to inhabit, the better. The vast majority will end up inside your filter, as they require a lot of oxygen to thrive. So make sure you have porous material in there, and always keep the filter running!

Step 2: Feed the bacteria

You’ll want to start with an ammonia concentration of 2 to 4 ppm. To get to this concentration, add 2 to 4 mL of pure ammonia per L of aquarium water (~1/2 Tbsp to 1Tbsp per gallon). This single dose should be enough to keep your bacteria fed for the entire duration of the cycling process.

Step 3: Monitor the parameters

Follow the instructions in the monitoring section, starting from step 2.

Emergency live-in cycling

If you already have animals in your aquarium and you’re unable to relocate them to a different, well-established tank while their current tank cycles, you may need to do an emergency live-in cycling. It will be stressful for your animals, but with careful monitoring and quick interventions, you can prevent chemical burns and secondary illnesses.

Step 1: Obtain the bacteria

Ideally, you’ll want to use seeded media for this, as it truly is the quickest and safest way to get your bacteria established. If that’s not an option, I highly recommend getting your hands on a bottle of Tetra SafeStart. This is, in my experience, the fastest and safest product on the market. Unfortunately it is nearly impossible to find in Canada. I would definitely carry it in the store if I could find a supplier!

If you do manage to get your hands on a bottle of TSS, you’ll want to do a big water change, then dump the entire contents of the bottle into your tank and leave it alone for two weeks. Note that you won’t be able to use conditioner during that time, as it interferes with TSS’s mechanism, according to Tetra. You’re not supposed to do water changes either, but obviously if your animals show signs of distress, you’re allowed to bend the rules a little. Just try to keep it to a minimum (you can do spot cleanings with a turkey baster, and refill to counter evaporative loss). Aside from that, just cross your fingers and wait.

If you can’t find TSS, just use any brand of bottled nitrifying bacteria. You can still keep your animals safe — it’s just going to be more work.

Step 2: Feed the bacteria

Your animals will take care of this step for you. Moving on!

Step 3: Monitor the parameters

If you’re using Tetra SafeStart:

Every two days or so, test the water for nitrites. If you detect 2.0 ppm of nitrites or more, do a 50% water change to prevent stalling (see the troubleshooting section for details). Make sure to use dechlorinated water — remember, you’re not allowed to use conditioner!

After two weeks, test the water for ammonia, nitrites and nitrates. If you detect no ammonia or nitrites, but nitrates are 40 ppm or more, congratulations! Your aquarium is cycled. Do a final water change and you’re done.

If you’re still getting ammonia or nitrites, start doing 50% water changes and adding conditioner once a day. You will need to do this until the tank is fully cycled — yes, it’s a pain, but it’s the only way to keep your animals safe! Start monitoring your cycle as described in step 3 of the monitoring section below.

If you’re using a different product:

Start doing 50% water changes and adding conditioner once a day. You will need to do this until the tank is fully cycled — yes, it’s a pain, but it will keep your animals safe!

Follow the instructions in the monitoring section below, starting from step 1.

Monitoring the cycle

Most guides have you testing ammonia, nitrites and nitrates with liquid test kits every single day until the aquarium is fully cycled. That is a lot of work! Here is what I recommend instead:

Step 1: Ammonia

Test for ammonia once a day until you get a reading of 0.25 ppm or more. Then, move onto step 2.

Step 2: Nitrites

Test for nitrites once a day until you get a reading of 0.5 ppm or above. Then, move onto step 3.

Note: if nitrites shoot up to 2.0 ppm or above, add conditioner. This will help prevent your cycle from stalling (more details in the troubleshooting section).

If you don’t record any nitrites for two weeks, consult the troubleshooting section.

Step 3: Nitrates

Test for nitrates once a day until you get a reading of 40 ppm or above, then test ammonia and nitrites. If they are both at zero, congratulations! Your aquarium is fully cycled. Do one last water change and you will be good to go.

If you’re getting 40 ppm nitrates or more, but are still registering ammonia, consult the troubleshooting section.

If ammonia is at zero but you are still getting some nitrites, you probably just need to wait a bit longer. Give it a couple days and test again.

Cycle troubleshooting

Problem 1: You’re not getting the first ammonia spike (all parameters are at zero)

You didn’t add enough ammonia to start the cycle. Are you trying to cycle your tank with fish flakes, raw shrimp or using animals who don’t produce much waste (e.g. snails)? Don’t do that. Use one of the methods described above, you’ll have a much easier time.

Problem 2: Ammonia is not converting into nitrites (high ammonia, no nitrites, no nitrates)

You don’t have enough nitrifying bacteria in the water. Add bottled bacteria, that will kick-start the process. Make sure your filter is equipped with biological media so they will have a space to colonize, and don’t forget to keep your filter running at all times.

Problem 3: Your cycle is stalled at the nitrite stage (little to no ammonia, high nitrites, no nitrates)

Your nitrites are too high. To avoid a bacterial die-off, you should be keeping the levels below 4 ppm. You can control the amount of nitrites by doing water changes (don’t forget to add conditioner!) A 50% water change will cut the amount of nitrites by half. If that’s still too high, do another 50% water change to cut that amount by half again. Don’t just change all the water at once, though — you want to keep some nitrites in the tank so that your nitrobacters don’t starve.

If you worry about water changes removing too much ammonia from your tank, you can use Seachem Prime to neutralize the excess nitrites instead. One dose will neutralize 0.5 ppm of nitrites for 24 hours. You can safely use up to 4 doses, even if there are live animals in the tank. Try to keep some nitrites for your nitrobacters to munch on, though! You don’t need to neutralize all of it, just enough so that it doesn’t reach 4 ppm. I recommend trying to stay below 2 ppm, just to be safe.

Problem 4: The cycle never seems to end (low ammonia, low nitrites, high nitrates)

You don’t have enough filtration capacity for the amount of ammonia that is being produced in your tank. Perhaps you are overstocking, are not using enough biological media, or your filter is inadequate.

Problem 5: Your cycle suddenly crashed (high ammonia, little to no nitrites, low to high nitrates)

Something killed off your nitrifying bacteria. Maybe your filter stopped running for too long (e.g. during a power outage). Maybe you forgot to dechlorinate the water. Maybe you treated your tank with an antibiotic. Whatever it is, you will need to restart the cycling process. If you’re lucky, some of the bacteria survived the ordeal. Follow the steps for live-in cycling and hope for a quick recovery!

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Filtration Basics

There are many models of filter, but they all rely on the same principles. There are three types of filtration: mechanical, chemical and biological. Understanding what these do will help you choose the right filter, save money on expensive filter media, and keep your aquatic pets safe from new tank syndrome.

Mechanical filtration
Most filter models rely on some type of mechanical filtration. Mechanical filtration is the process of physically trapping debris that are floating in the water, so that you can remove it by hand. This is done by forcing the water through an absorbant, porous material such as a sponge, filter pad or floss. Any particles too big to fit through the pores will stick to the material, which will eventually clog and need to be rinsed out or replaced.

Materials will large pores will last longer without clogging, but will let many small particles through. We often refer to those materials as pre-filters, because they are typically used as the first step of any multi-step filtration system. By removing the largest chunks, it allows the rest of the system to do a better job without clogging right away.

Materials with small pores will trap fine particles, but this also means they will saturate easily and need to be replaced often. We tend to refer to these materials as water polishers, since they help to make the water look much clearer. These are typically used as the last step of any multi-step system.

Chemical filtration
This type of filtration relies the chemical properties of carbon or other materials with high affinity for the types of molecules that we want to remove from the water (ammonia, heavy metals, etc.) As water flows through the substance, those molecules bind to the carbon atoms (or whatever the chemically attractive substance is). Once saturated, the material needs to be replaced or recharged.

Because we often use carbon cartridges to filter our own drinking water, chemical filtration is what most people think about when they think of water filtration. You might be surprised to know that this type of filtration, while useful, is not actually essential in an aquarium. Unless you are trying to remove leftover medication from the water, or you are dealing with a major ammonia crisis, it’s perfectly fine to omit it.

In fact, chemical filtration can sometimes cause more harm than good. Some of the most commonly used substances, such as carbon, begin to leech harmful substances back into the water once it reaches its saturation point. For this reason, it absolutely must be changed once a month. If you’re as lazy as I am when it comes to filter maintenance, you’ll probably want to look into different options. Carbon use has also been linked to hole in the head disease (HITH, or head and lateral line erosion, HLLE) in certain species of fish.

Biological filtration
This type of filtration is absolutely essential to the survival of your aquatic pets. A healthy aquarium isn’t just a glass box filled with water — it’s a whole ecosystem, full of living organisms with important roles to play. The term “biological filtration” refers specifically to the most crucial of these organisms: two groups of bacteria that, together, process harmful nitrogenous waste into a much less toxic form. You can read more about this process here: Introduction to the Nitrogen Cycle (or, Why You Killed That Goldfish When You Were Little). For now, let me just say that without these little guys, we wouldn’t be able to keep aquatic pets alive in our homes. Hobbyists call these little magic helpers BBs (short for beneficial bacteria).

To be able to perform their important job, BBs need to make a cozy home for themselves inside your filter. They inhabit porous materials like sponges, bio-balls and ceramic rings. Collectively, we refer to these substances as bio media. The process of helping the BBs colonize their new homes is called cycling, and it is the most important thing you need to do before bringing any new aquatic animal home. You can read about proper cycling methods here: How to Cycle Your Aquarium.

Keep in mind that bio media should never be replaced. If it’s especially dirty, you can rinse it by swishing it around in a bucket filled with aquarium water. Never rinse your bio media under the tap, as BBs are sensitive to chlorine. Don’t squeeze the sponges, either!

If your filter only accepts disposable cartridges, make sure to always leave an old cartridge behind the newer one to give the bacteria a chance to migrate (or better yet, get a better filter!)

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Introduction to the Nitrogen Cycle (Or, Why You Killed That Goldfish When You Were Little)

Most first-time fish owners kill their pet within a month of bringing it home. This is because most pet stores fail to educate their employees and customers about the nitrogen cycle, a crucial component of the aquatic ecosystem. This beautiful example of commensalism (a symbiotic relationship that benefits all of the organisms involved) is what makes life sustainable for aquatic animals.

The first thing to understand is that an aquarium is not just a box with water in it: it is a miniature replica of the ecosystem that allows life to flourish in our rivers and oceans. The water is full of microorganisms, and some of them have extremely important roles to play.

Meet the nitrifying bacteria:

In the aquarium, just as in nature, animal waste and decaying organic matter constantly release ammonia (NH3) into the water. Ammonia is highly toxic — even a trace can be harmful to most aquatic species. Luckily, bacteria like nitrosomonas are able to use the oxygen present in water to convert ammonia into a less toxic compound: nitrite (NO2).

Note that “less toxic” does not mean “harmless”. Animals tolerate nitrites a bit better than ammonia, but as it builds up in the aquarium (or any other body of water), it can still lead to burns, illness and death. Thankfully, that’s when nitrobacters come in to save the day. Using oxygen, these little guys convert nitrites into nitrates (NO3), which are far less toxic than the other two compounds.

In nature, nitrates don’t really have a chance to build up because plants and algae use them for growth. If you keep plants in your aquarium, they will “eat” some of the nitrates for you. The leftovers will become algae food, which is why you may notice an algae bloom if you haven’t changed the water in a while.

If nitrates keep climbing, the plants and algae may not be able to keep up, and your animals are likely to get sick. This is where you come in! Regardless of the species you are keeping, you should be doing partial water changes at least once a week. It’s a good idea to test for ammonia, nitrites and nitrates once in a while to make sure that your microorganisms are doing their job and keeping your aquarium water safe.

Which brings me to the main point of this post: brand new, clean aquariums are unsafe.

More specifically, the problem lies with the filter. When you first obtain it from the store, the filter is bacteria-free. This means you need to install them yourself and give them time to make themselves at home before you can safely add any animals. We call this process cycling. It takes a while and it’s kind of tricky, so people who don’t understand how important it is tend to skip this step. Then, as ammonia builds up, their animal’s health begins to deteriorate. This is such a common phenomenon that it has been given the name new tank syndrome.

Now that you know why cycling is so important, here’s how to do it properly.