Why We Favor Fishless Cycling with Cured Live Rock

Cycling with live rock is an easy and reliable method to establish the biofilter in your saltwater aquarium

Cycling with live rock is an easy and reliable method to establish the biofilter in your saltwater aquarium

In years past, the most common method for establishing biofiltration in marine aquariums was to introduce some hardy, rugged fish to the system as an ammonia source and wait for the cycle to get established before adding more livestock. The usual go-to fishes for this purpose were damsels. While this cycling method does work, here at Saltwater Smarts, we favor so-called fishless techniques, such as cycling with cured live rock (my preferred approach—though there are others).

When added to a new aquarium, cured live rock typically releases just enough ammonia to get the cycle started through the additional die-off of encrusting organisms. That modest die-off combined with proper tank conditions—excellent water movement and oxygenation—virtually ensures the porous rocks will soon support a good population of aerobic nitrifying bacteria, allowing gradual/incremental stocking to commence.

But why is this method any better than adding a few hardy damsels?

The cruelty factor

There’s a good reason “hardy, rugged” fish were used to cycle tanks—more delicate, sensitive species were unlikely to endure the process. But just because damsels may (some don’t) be able to survive exposure to a succession of toxic chemicals doesn’t necessarily mean it’s humane to put them through it, especially when other means of cycling are available.

The territorial dominance factor

Though there are noteworthy exceptions (such as Chrysiptera parasema), damsels tend to be highly territorial and aggressive, so adding them to an aquarium first turns the appropriate order of introduction (from least aggressive to most aggressive) on its head.

If a damsel introduced for the purpose of cycling claims the entire tank as its territory—as is often the case—adding any other fish to the system thereafter may prove problematic if not downright impossible. Your only realistic option in that circumstance is to capture and remove the damsel(s), which may necessitate tearing apart all the aquascaping and completely disrupting your new system.

The fish disease factor

If damsels used for cycling happen to be carrying disease, the tank is now potentially infected along with any fish added in the future. Of course, you can avoid this issue by quarantining/treating the damsels in a separate system before cycling your display tank, but that sure seems like a lot of unnecessary hassle just to get your biofilter established.

The simplicity factor

Best of all, live rock cycling is about as straightforward and uncomplicated as you can get. You basically just add the live rocks to your system and perform tests to monitor the ammonia and nitrite levels until both are no longer measurable. Then it’s safe to start adding livestock. It’s a method that works reliably for me every time.


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. Matt Bowers (Muttley000) says

    Thanks for writing this. I hate to hear of cycling with fish for the very reasons you list, and appreciate you bringing it to the forefront!

  2. Tony Guido says

    Would this method work with the dead or artificial rock available? Some reefers don’t like the pests that can be introduced from live rock. I’m assuming you would still have to add some live rock. Some reefers suggest adding shrimp. Your thoughts on this appreciated.

    • Jeff Kurtz says

      Hi Tony! If you use dead or artificial rock, you’d need to introduce ammonia by some other means to get the cycle going and encourage beneficial bacteria to colonize the rocks. Adding flake food or something like shrimp to the tank and allowing it to decompose is one way to achieve that. Another is adding household ammonia (with no dyes, perfumes, or other additives).

  3. Hi Jeff,

    I’ve started cycling with live rock, and discovered a few small crabs this week (week 2). I’m going to assume it’s a good thing, but would love to know if not so I can remove them asap.


    • Hi Wayne! Though there are exceptions, crabs tend to be opportunistic omnivores, which means they have the potential to be destructive in a reef tank if they decide to sample the local invertebrate “wares.” To be on the safe side, I’d recommend that you remove the crabs if you’re able (or at least keep a very close eye on them and be prepared to move them if problems arise). Some hobbyists like to transfer crabby stowaways into a sump or refugium so they can still enjoy them without putting their prized livestock at risk.

  4. Hi Jeff,

    I have had a 75G setup for years. I would like to setup a permanent 20G quarantine tank. Can I use some of the live rock from my display? If so do I throw a raw shrimp in with it?

    Thank you.

    • Jeff Kurtz says

      Hi Brian! You can definitely use live rock from your display to provide biofiltration in your QT. I would recommend keeping the rocks in the display until right before you introduce a specimen to quarantine. With this approach, you shouldn’t have to worry about using raw shrimp because the rocks would already be colonized with nitrifying bacteria ready to convert the ammonia produced by the quarantined specimen.

      The only drawback to this method is that if the quarantined specimen proves to be sick and needs treatment, you may not be able to return the rocks to your display. If I know far enough ahead of time that I’m going to be adding a specimen, I’ll sometimes put some chunks of rock in the sump of my DT several weeks ahead of time and then move them to the QT when I acquire the specimen. That way I don’t disrupt the aquascaping in my DT and if I end up having to sacrifice the rocks, it’s no big deal.

      • Thanks for the response. Yes that is a drawback. What if I was to use some sort of sponge instead? Like the sponges for an overflow box, or the little ones that come with powerheads. Any chance if I put those in the sump of my DT that might seed my quarantine tank? If so, about how long should it be left in the display tank? If something like that would work then I wouldn’t have to set up a QT permanently, I could just set it up the couple times a year I might need it.
        Thanks again for your help.

        • Jeff Kurtz says

          You can use sponge as well, but you have to make sure the water is actively flowing through it in both the DT and QT so the nitrifying bacteria get plenty of oxygen-rich water. In other words, you don’t want the sponge just sitting in slack water somewhere in the system.

          Also, you’d want the sponge to be of a fairly good size to support adequate bacterial colonies. Some of the smaller sleeve-type (for lack of a better descriptor) sponges used in overflows might not be up to the task all by themselves, though they will still contribute.

          One way to do this is to keep one of those old sponge filters operating in your DT sump and then move it over to the QT as needed.

          In any case, quarantine biofiltration tends to be somewhat tentative, so whatever method you choose to seed the biofilter, keep testing the water and be prepared to do water changes if you get an ammonia spike at any point in the process.

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