How to Cycle a Saltwater Tank in 24 Hours

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Did you know that it’s possible to have a marine aquarium cycled within a day of setting it up? It requires a few special things, but it’s possible and will keep you from staring at an empty tank for weeks while you wait for things to build up.

Ready to learn how to cycle a saltwater tank in 24 hours? Great, let’s dive in!

saltwater tank


What You Need

You’ll need a few extras when you’re getting the tank set up. After all, we’re attempting to kickstart a biological process that normally takes a few weeks and compressing that time into a 24-48 hour period.

You’ll need everything normally required for a saltwater aquarium, but the following extras are required to pull this off:

  • Used filter media from an established marine tank
  • A bacteria supplement like BioSpira Instant Ocean
  • An aquarium test kit
  • Live rock, at a minimum of 1 pound per gallon of water
  • An air pump appropriate for tank size
  • Live sand for the substrate

Most of these are easy to acquire, but used filter media can present a challenge if you don’t have friends with saltwater tanks or your own tank already.

I’m not aware of any store selling used media, the rough equivalent is the bacteria supplement we’ll be dropping in but we’re trying to speed things up as much as possible.

Lacking a better source try asking at your local LFS. A lot of mom-and-pop shops will hook you up with used media for free or at a very minor cost. Most big box stores will shoot you down, and you should be supporting your local specialty shops anyways.

Please note that not every LFS is suitable. You’re better off just skipping the media if all of your local stores have unhealthy fish. Inspect their marine tanks carefully for any sign of illness before acquiring any used media from them.

You can also try and acquire some substrate from an established tank if you don’t want to use live sand. The substrate of a tank contains a lot of bacteria once it’s been running for a while. It can be a pain to acquire if you don’t have friends in the hobby, so don’t consider it essential.

If you can, try acquiring some water from an established saltwater tank as well. It’s a minor factor, but our goal is to ramp up bacteria production as quickly as possible.

A filter that’s been used in an established tank is ideal, just transfer it over once your tank’s water has been dechlorinated. That said, it’s rarely a practical solution unless you planned far ahead.

The Holy Grail of this task is a fully cycled canister filter that’s been running in an established tank. If you can get your hands on one then you’re almost guaranteed to succeed at quick cycling your tank.

1. Set Up the Tank

The first thing to do is to set up the tank as normal. Add your substrate, your water/salt mixture, and live rock to the tank. Arrange everything to your liking as you would normally.

At this stage, it is essential to make sure that you don’t have any chlorine in the water. Chlorine is added to tap water to kill bacteria, making it just a bit counterproductive to our end goals. You can use SeaChem Prime (my personal favorite) or whichever brand you normally use.

Hook your air pump up at this stage. Consider running more air than you normally would. The bacteria that live in the water column and your filters thrive with extra oxygen in the water.

If you have a seeded substrate, then place it underneath the new substrate when you’re getting things set up.

Add the water and perform any alterations you need, such as pH buffers. Your pH should be at the low end of the acceptable range, 7.6 for a standard marine tank or 8.0 for a reef tank. Bacteria grow better in the 7.0-8.0 pH range, but we still need to keep things acceptable for invertebrates and corals.

Our goal here is to create an inviting environment for the bacteria before you begin adding the majority of the bacteria that will be used in the tank.

Do not add any fish or invertebrates yet. We’re going to wait until we’ve taken a few more steps towards quick cycling before we put them in.

2. Add Used Filter Media

Get your filters set up as normal, but use the used filter media instead of brand-new stuff. You can set the new stuff aside for the next filter change.

Used activated carbon or bio-filtration media like BioBalls are the best. They have an incredible surface area compared to their size, which means a much larger colony of bacteria. Sponge segments can be used but aren’t nearly as efficient.

If you do have sponge segments, then it may be best to place them in a sponge filter at the bottom of the tank instead of in a canister or HOB. You can tie this in with your air pump instead of running a separate piece of equipment.

After introducing your used filter media, you’ll want to turn on the filters and let everything run for an hour or two before moving on.

3. Add Your Bacteria

You should now have a decent environment for your bacteria to begin multiplying. The basic idea here is that you’re kickstarting the cycle by dumping in a bunch of nitrifying bacteria, forcing the chemical system in the tank to get close to equilibrium as soon as possible.

Just read the bottle and dump in the amount it says for the size of your tank. I’ll usually add ~150% of what the bottle says when trying to boost the speed of a cycle. It’s a little bit more expensive, but I’ve found its more likely to be successful.

I’m not aware of anyone having done a brand-by-brand analysis of the different bacteria formulations available. Most of the information available is anecdotal, which is why I recommend using BioSpira Instant Ocean since I know it’s worked for me. It’s also one of the few that seems to be formulated specifically for marine environments.

Your bacteria are an essential part of the internal ecosystem of the tank. At this point, we should have a lot going on when it comes to bacterial life.

Live rock comes with its own bacteria, used media and substrate add more, and the liquid culture is a huge kick to the population inside.

We’re creating an ideal environment, but we need to take care of one more variable.

4. Get the Temperature Up

Heat is the final piece of the puzzle and one that’s often overlooked.

We all know that bacteria grow best in wet, warm environments. We’ve got wet taken care of just by virtue of an aquarium’s nature, but we’ll want to run things a bit warmer than normal for now.

Since marine tanks usually run in the 75°-82°F (22°-29°C), we’ll want to be at the upper end of things. We can’t go much higher since marine systems need a stable temperature. Set your heater to bring the tank up to 82°F for now.

Now comes the “fun” part of the process.

5. Add Fish and Begin Testing

At this stage, you’re fine to add 1 or 2 fish. They should be relatively hardy, fish like clownfish, damsel fish, or chromis. They’ll immediately begin adding nutrients to the water for the bacteria to eat, and it’s not a bad idea to feed them once they’ve been acclimated.

Fish release ammonia as a waste product through their gills, but we need to monitor the tank and make sure that everything stays within a reasonable range during this quick cycle. If you were able to get everything then the tank should be stabilized within 24-48 hours, and it will be safe for hardier saltwater fish immediately.

Begin testing every few hours for the next couple of days. You’ll most likely see small spikes in ammonia and then nitrite, followed by a corresponding increase in nitrate levels as the bacteria begin to break down the waste in the tank.

There are no guarantees, the more used media/filters and live sand/rock you get in there the better off you’ll be. If you were wise with your fish choice then you’ll be fine even if there are some small spikes in the toxic elements of the nitrogen cycle.

Just keep monitoring the situation and take action if things don’t look right. I’ve yet to see it happen with a proper run at a fast cycle, but if ammonia or nitrite levels start looking high and your fish start looking ill then you’ll need to rethink how you’re doing things.

After 48 hours most tanks that have undergone this process should have negligible levels of ammonia or nitrite. Once those levels reach zero, then you’ve successfully cycled your tank and you’re ready to kick back and enjoy!

You can bring the temperature back down and turn off any extra airflow you had coming into the tank after 3-4 days.

Cycling Quickly and Safely

Learning how to cycle a saltwater tank in 24 hours is just a matter of knowing what conditions breed bacteria and how to get a large initial seeding. With the help of a bacterial supplement, some live rock, and material from cycled tanks you can get your tank up and running without the traditional weeks of waiting.

So, are you ready to try your hand at managing the bacterial cycle?

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