How To Cycle An Aquarium

Not cycling your aquarium is one of the most common mistakes beginners to the aquarium hobby make. Due to a lack of interest in informing new hobbyists at major stores like PetSmart, very few beginners take the time to properly cycle their first aquarium.

It doesn’t matter whether you’re beginning with a one gallon aquarium or setting up a one thousand gallon aquarium, properly cycling should always be the first step in preparing your new aquarium.

Instead, most first time aquarists end up unknowingly cycling their aquarium when they add their first fish.

The majority of aquarists will lose fish due to not properly cycling their aquarium.

When you add your first fish to an aquarium that hasn’t been cycled, the ammonia from this fish leads to the growth of the necessary bacteria. While you might be able to cycle your aquarium this way without fish dying, your fish will still suffer from the high ammonia levels in the aquarium. By properly cycling your aquarium before introducing your fish, you can remove the risk of your fish dying, while keeping them healthy. As a result, in this article, we will, in detail, provide a guide on ‘How To Cycle An Aquarium’.

Aquarium Nitrogen Cycle Steps

To describe how to cycle an aquarium, we must begin by discussing the aquarium nitrogen cycle and its steps. Simply put, cycling your aquarium consists of establishing the bacteria in your aquarium’s biological filter. These bacteria primarily live in your filter media, but can also live on aquarium surfaces and in gravel.

As a happy aquarist, every day you wake up and feed your fish some food. When your fish consumes this food, it digests it and turns it into waste. That waste then comes out of the fish in the form of ammonia. That ammonia, which can also come from excess food and plant remnants rotting, is then transformed by Nitrosomonas bacteria to nitrite. Toxic gases from anaerobic bacteria in the dirt can also produce ammonia and nitrite.

Nitrobacter bacteria then takes that nitrite and turns it into nitrate. That nitrate can either build up in your aquarium, where it has to be removed by water changes, or turn into plant fertilizer, depending on whether you have live plants in your aquarium. Cycling your aquarium is the process of providing an ammonia source, so this bacteria can grow, without subjecting your fish to the toxic levels of ammonia or nitrite that occur before the bacteria has completed growing.

At Freshwater Central, we highly recommend using live plants to remove nitrate from your aquarium. As the plants consume the nitrate and grow, you can then trim the plants, to remove nitrate from your aquarium. By having plants, not only do you allow yourself to make water changes less often, but you also provide a vibrant environment for your fish that mimics the natural environment and provides them with places to hide.

To quickly summarize the process, properly cycling your aquarium involves introducing an artificial source of ammonia into the aquarium. Slowly adding additional ammonia to the aquarium will result in bacteria growing that converts the ammonia into nitrite. Once your nitrite level increases, additional bacteria will start growing to convert this nitrite into nitrate. At this point, you can stop adding artificial ammonia, do a large water change to bring down the nitrate level, and add your fish into the aquarium.


Before we begin the process of cycling your aquarium, you will need two pieces of equipment. The first is either some shrimps from your local grocery store  or ammonia chloride depending on the form . As we saw above in the aquarium nitrogen cycle, decaying animal matter produces ammonia. As a result of this, we will add ammonia to the aquarium by either will be placing the shrimp inside or pouring in the ammonia chloride.

The second is a test kit capable of telling you the ammonia, nitrite, and nitrate levels of your water. I recommend the API Freshwater Master Test Kit. This kit has everything you need to test the ammonia, nitrite, and nitrate levels of your aquarium along with the pH levels. It can be bought at your local fish store  or online at Amazon for $30 or less.

Method 1 – Shrimp

The first method of fishless cycling your aquarium involves introducing shrimp as the artificial source of ammonia in your aquarium. Take the shrimp you purchased from your local grocery store and cut it up into little pieces. Drop a piece or two into your aquarium, and leave it there as it slowly decays, though it might unfortunately be unsightly.

Now that you’ve begun the cycle process, test your water on a daily basis and look for a spike in ammonia. Within a week or two you should see a the spike in ammonia peak and then begin to go back down. Whenever your ammonia drops back to below 1 ppm, add in another piece of shrimp, but remove some pieces of shrimp if your ammonia gets above 3 or so ppm. Letting your ammonia level get too high can actually kill the healthy bacteria that are part of your cycle.

Looking at the levels, you can see that after the first two weeks your nitrite will begin to spike. I recommend incorporating daily nitrite testing in your cycling regimen after the first week and looking for a spike in your nitrite levels. As your nitrite levels go above 2 ppm, stop adding pieces of shrimp, until your nitrite levels drop back to down to below 2 ppm. Once the levels drop back down to less than 2 ppm, you should see your nitrate levels begin to increase, and you can add additional shrimp every time nitrite drops below 1 ppm.

One last thing to keep in mind is as your aquarium cycles, you might see the water turn cloudy as bacteria grows. This is to be expected, don’t worry about it! Eventually, you want to get to a point where you can add a piece of shrimp every 3 or so days and not see your ammonia or nitrite levels spike at all in daily testing, while your nitrate levels steadily increase. Once you reach this point, your aquarium is now cycled! Feel free to now remove all the decaying pieces of shrimp.

Now you’re ready to go to the last section of this guide for learning how to add fish!

Method 2 – Ammonia Chloride

The second method, and my preferred method of cycling due to the fact that it’s much more reliable, is by introducing ammonia through ammonia chloride. The aquarium industry has progressed past when I first started, and now there exists some specific products that market themselves as an ammonia source for fishless cycling. Your local fish store likely has some of these products you can use.

However, if you want to save some money, you can go buy a source of ammonia chloride from your local fish store. Look for a bottle of ammonia that only has ammonia and water listed on the ingredients. When you find a bottle that you think is suitable, shake it up and see if it foams at all on the surface. If it doesn’t then you’re good to go, this bottle is your source of ammonia!

Now the method to cycle your aquarium with this source of ammonia is mostly the same. Add some of this ammonia chloride to your aquarium (start small, this ammonia is often fairly concentrated) until your aquarium levels reach 4 ppm. Start testing your ammonia and nitrite levels in both the morning and the evening and every time your ammonia levels drop to below 1 ppm, add some to get it back to 4 ppm.

At the same time, keep an eye on your nitrite levels. If they increase to above 2 ppm, stop adding ammonia until they subside back down to below 2 ppm. Eventually, you should see both your ammonia and nitrite levels decreasing as your nitrate levels begin to increase. Keep continuing to add ammonia whenever your ammonia levels drop to below 1 ppm as long as your nitrite levels are below 2 ppm.

Continue this process until you can get your ammonia levels to 3 ppm and then have these levels disappear into nitrate within 48 hours with no spike in nitrite throughout the entire time period. At that point, you have finished cycling your aquarium!

Be careful not to add ammonia at too high of a concentration, else you risk killing the bacteria already created. 

Now you’re ready to go to the last section of this guide for learning how to add fish!

Adding Fish To Your Cycled Aquarium

Now that you’ve cycled your aquarium, it’s time to begin adding fish. Despite the fact, that your aquarium is cycled, I still recommend that you slowly add fish. Start by adding any smaller individual fish you have planned. For example, if you plan to have a school of 6-8 neon tetras or panda corydoras, start with those as they tend to have a much lower bioload.

Remain vigilant and continue to check both your ammonia and nitrite levels on a twice daily basis. If your levels spike, add in some Seachem Prime, or do a large water change, to get them back down. After a week or two you can continue to add additional fish. I recommend following a rule of 4-5” of fish per week when stocking your aquarium.

That means, if you want to add a single larger fish that’s 4” long, add it by itself on its own week. However, if you want to add a pair of 2” guppies, you can add them together in the same week. This rule will allow you to stock your aquarium, while keeping from stocking it too quickly and causing a spike in your ammonia or nitrite levels. Soon your aquarium will be fully stocked!

Aquarium Cycle Starter

As you browse through your local pet store, you might notice some products marketing themselves as “aquarium cycle starter” or “aquarium instant cycle”. Basically, these are all products marketing themselves to people who are looking for how to speed up their aquarium cycling. The general idea behind these products is the manufacturer puts bottles up the bacteria your aquarium needs, and sells it to you to pour into your aquarium and speed up the cycle.

At Freshwater Central, we feel that for most people these products are a waste. If you do choose to use these products, don’t trust the claim that you can pour them into your aquarium, and your aquarium will be suddenly ready for fish. Cycle your aquarium using one of the two methods above, but pour in these products after you see that first ammonia spike, and hopefully the bacteria can help accelerate the cycling process.

Once you reach the end point in one of the cycling methods, i.e. you can either add a piece of shrimp every 3 days or get your ammonia levels to 3 ppm and have them decrease back down over 48 hours with no spike in nitrite, you can now consider your aquarium cycled.

Cycling Aquarium With Fish

I’m hesitant to include this section at all, given that cycling your aquarium with fish isn’t the best for it. However, given the number of people who might end up doing this, I think it’s an important section to have nonetheless in a thorough guide.

The key to cycling an aquarium with fish is taking everything incredibly slowly. Start by adding a single fish you want to stock, preferably a smaller fish and one that isn’t a schooling fish, since adding a schooling fish by itself will stress it. Leave this fish alone in your aquarium, feeding it daily, while you keep a close eye on your ammonia and nitrite levels. Anytime your ammonia levels go above 0.5, do a large water change.

After two weeks, add another fish. For the first 2 months, you don’t want to add more than 1” of fish every 2 weeks. Keep a close eye on all your fish for any sign of stress or ammonia burns (red streaking across body, fish gasping for air, purple or red gills). If your ammonia levels go above 0.5 ppm do a 50% water change. Eventually you should get to a point, where your ammonia levels don’t increase at all as you add fish. You can then double the rate you add fish until your aquarium is fully stocked.


Cycling your aquarium can be a long and time consuming process, especially for someone excited to add fish to their brand new aquarium. For many more people, they simply have never heard of cycling before, it is not information normally shared by your local Petco or Petsmart store.

However, despite this, cycling your aquarium is essential to the health of your fish and you should take the time to properly cycle every new aquarium. I look forward to reading your thoughts on this important subject in the comments below.

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