The easiest (safe) one-week aquarium cycle
There are a lot of guides on the internet about cycling aquariums. This is one of them.
We take a different approach, though, and make it incredibly easy and incredibly quick. We’ve used this method to cycle 40+ tanks without ever losing a single fish or a single plant. Cycling usually takes anywhere from 2-10 weeks and is generally an annoying, painful time where you’re dying to see your aquarium thrive but you have to just stare at an empty tank and constantly test it. This method involves almost zero testing, zero calculating, and even allows you to add fish immediately. This method works on tanks of any size, from 1 gallon to 1000.
To start, let’s quickly cover what cycling is, and how to properly do it.
What is cycling?
When anything organic breaks down underwater, it creates ammonia. Ammonia is extremely toxic to most living things, and will very quickly kill your fish and plants as they create waste. The solution to this issue is by utilizing millions of bacteria in your ecosystem, that consume ammonia and eventually convert it to nitrates, which can be consumed by plants as fertilizer and isn’t toxic to fish.
The bacteria will naturally begin growing after dechlorinating your water, but usually takes quite a while to multiply enough to actually stabilize your tank. This is called cycling a tank, and once the bacteria has grown enough to stabilize the tank and support life, the tank is considered cycled. The bacteria generally grows in your filter, and the more surface area for the bacteria to grow, the better. Sponges and biomedia are the two best places for it to grow. We like using lava rocks, which are cheap and have tons of surface area for bacteria.
The important things to remember about this bacteria is that it is aerobic, and that it needs to consume ammonia. Aerobic means it requires oxygen to live, which is precisely why this bacteria usually lives in your filter where there’s lots of water flow and oxygen. This is precisely why stagnant bodies of water always look and smell gross, while flowing water is usually very clean. The second factor is equally important, which means that if you just hook up a filter to an empty tank full of water, you’ll have a lot of trouble actually cycling the tank. A consistent amount of ammonia for them to feed on will help them multiply very quickly.
Finally, many people leave out that while the bacteria does need ammonia to live, it will also be killed with high levels of ammonia. Generally, anything over about 1ppm of ammonia is toxic to your cycle and can cause your cycle to slow or even crash.
The One-Week Cycle
What you need
A fully set up aquarium, including substrate, hardscape, a heater, a filter, and a light.
A decent amount of fast growing plants, usually enough to fill about 30% of the substrate or more.
A small number of hardy fish, we prefer bristlenose plecos for any tank 10g or more or snails for smaller ones.
Dirty filter media from another aquarium (You can order some for cheap!)
An easy way to conduct water changes
Regular aquarium chemicals, such as fertilizer and dechlorinator. You don’t need bottled bacteria or ammonia remover.
A test kit capable of testing ammonia (Optional, but recommended)
Setup
First thing’s first, just set up your tank! I usually recommend about 2” deep of substrate for easy planting and because bacteria lives in substrate, so more substrate means a more stable tank. Add some hardscape and anything else you’d like in your tank, and make sure you have regular equipment such as a filter, heater, and light. Check out our lighting guide for recommendations on that.
You’ll want lots of plants. Plants not only give your fish enrichment and entertainment, but they’re crucial in keeping your tank stable because they can absorb nitrate and even ammonia to a small degree. Add plants as soon as you start your tank, and add fertilizer to the water because they won’t have any other nitrates to feed off of. We recommend using fast growing stem plants or floating plants- epiphytes such as Anubias or Java Fern grow so slowly that they don’t really absorb much nitrate.
You’ll also want an easy way to conduct water changes, because you’ll be doing them daily. The python system is a fantastic option, or simply using a long hose to a bathtub with a siphon. Whatever it is, you’ll probably want something easier than just buckets, especially for large tanks.
Start the cycle - Day 1
With your tank set up and filled, and the filter heater and light running, add dechlorinator and your cycle has begun! In a few hours, your water will be relatively stable and safe to add fish to.
Remember that the bacteria needs to eat ammonia, and you’ll need a consistent ammonia source. We’ve found that adding food to decompose or dosing liquid ammonia can work, but adding a small amount of livestock to the tank works much better and faster since fish waste already has bacteria in it. We usually recommend adding a bristlenose pleco or two to any tank 10g or larger, or just a few mystery snails or guppies to smaller tanks.
After adding some livestock to the new tank, your tank now has a consistent ammonia and bacteria source. Next, you’ll want to add some preexisting bacteria- this is the most important step. It takes far too long to wait for the bacteria to grow naturally, so you can speed up the process by adding bacteria from another tank. Anything in another running aquarium’s filter will work, the nastier the better. Squeeze in loads of fish poop and filter gunk straight into the water of your new tank, and your filter will quickly pick it up and get the bacteria-filled gunk mixed all into your filter’s media.
If you don’t have another tank to get filter media from consider sourcing it from other hobbyists. Some local fish shops will happily squeeze one of their filters into a bag for you, or you can order some pre-cycled filter media from shops like ours online!
This new bacteria will consume the ammonia being produced by your fish, and very quickly multiply within your filter and your substrate.
Mantain the cycle - Days 2-6+
With the cycle started, filter gunk added, and livestock added, your tank is on track to be quickly cycled.
Now, your top priority is not killing your livestock and bacteria. This means keeping ammonia low. If you’re using a test kit, you want to shoot to keep ammonia between 0 and 0.5ppm. This concentration of ammonia will be relatively non-toxic to your livestock, while giving your bacteria enough food to continue growing.
You’ll accomplish this by performing daily, or twice-daily 80% water changes. I find usually once-daily is perfectly fine in most cases, unless you have a lot of ammonia in your tank such as ammonia-rich aquasoils or you added a lot of stock. Drain the tank almost the entire way, leave maybe an inch or two of water, then fill it all the way back up with clean, dechlorinated water. Don’t clean the filter.
When the cycle is finished - Day 7+
After roughly a week, your cycle should be just about finished! You’ll know your cycle is almost finished when you begin seeing algae. Algae can only grow with the presence of nitrate, and nitrate will only be present once the aquarium is cycled and the bacteria is able to convert ammonia to nitrates. At this time, you’ll also begin seeing new growth on your plants.
Once you begin seeing algae, this doesn’t mean your tank is safe yet. It DOES mean your cycle is working and it is almost complete, but you should still continue doing daily water changes for a few days more, just to be safe. Again, if you have a test kit, at this point it’s worth testing daily until you see zero ammonia present in the water. If you don’t have a test kit, just continue doing daily or semidaily water changes for a few more days, and keep a close eye on your livestock in case they begin acting weird.
The algae cycle
After day 7ish, your tank should be almost completely cycled. After about 10-14 days, you’ll begin the dreaded algae cycle - an inevitable stage of cycling where loads of algae will begin forming. This is totally normal and nothing to worry about, and it’s very important that you don’t make any changes. Don’t turn down your lighting, don’t remove all your plants. Just leave your tank and continue performing frequent water changes. Once you begin seeing a lot of algae, this is when it’s safe to start adding lots more fish, and your tank should be able to keep up without much of an issue.
The “Algae Cycle” happens because your tank IS cycled, but isn’t fully mature. See, while we touched on the aerobic bacteria earlier that convert ammonia to nitrates, there’s actually a second type of bacteria, the anaerobic bacteria. Bacteria that thrives in the absence of oxygen. Anaerobic bacteria lives in the deepest nooks and crannies of your filter media, and is super useful because they converts nitrates to simply nitrogen gas, which is safely released from the water.
During the algae cycle period, your tank usually has a lot of nitrates. Your plants are still small and growing, and your filter has very little anaerobic bacteria which takes longer to form. Lots of nitrates mean lots of algae, hence why you’ll have a lot of algae growing. We usually daily dose algaecides such as Seachem Excel to slow the algae, and trim algae-covered old growth on plants to keep plants growing well. Again, do not make any changes to your lighting or tank. Turning down your light will cause your plants to grow worse and cause an imbalance, so when your tank is finally rid of algae and you turn your light back up you’ll just have another giant algae bloom. This time is where your tank balances and stabilizes as an ecosystem, and you don’t want to completely change your ecosystem. Think how much a rainforest ecosystem would change if the sun suddenly shined for only 3 hours a day.
The algae cycle usually lasts about 2 weeks. I try to do semi-daily water changes to keep nitrates low, and dose algaecides to fight the growth of algae. After a week or two the algae finally begins to die down, and plants begin to grow in much better. As your plants grow in more, they’ll do much better at absorbing nitrates, and your filter will do a better job at breaking down nitrates as well. And finally, at this point, your tank is perfectly cycled and stable.
