Fallow periods: Going Fishless


Dr. Fish
Going Fallow
The reason to go fallow (fishless) is to eliminate a fish disease from your DT (display tank). Going fallow works because you are denying the disease a fish host to feed upon, essentially starving it to death.

In order to go fallow you must remove ALL fish from your DT. If just one fish is left behind, even a “disease resistant” species, then going fallow is for naught because the disease will continue its life cycle. Corals/inverts can be left in the DT, as those are not capable of hosting - although some are capable of “housing” the encysted stage of certain parasites for a period of time (see “Fallow Periods” below). It is important to continue to feed your corals/inverts while going fallow, and also put a pinch of flake or pellet food into the DT every 2-3 days to feed nitrifying bacteria in the absence of fish poop. Some hobbyists even resort to dosing phosphate & nitrate if those parameters fall too low. Continue to do everything normally with your tank whilst going fallow; although you may wish to go lights out if you are running a fish only system (just don’t forget to feed that bacteria!)

Fallow Periods - Below is the general consensus fallow periods for all diseases that require it. In most cases, it is the longest known time period that the encysted stage can survive on corals, inverts, rocks, substrate without a fish host to feed on. The fallow period starts when the last fish is removed from the tank.
  • Black ich (turbellarian worms) - 4 weeks
  • Brooklynella aka “Clownfish disease” or “Brook” - 6 weeks
  • Flukes (monogenean worms) - 4 weeks
  • Ich (Cryptocaryon irritans) - 76 days
  • Uronema marinum - No fallow period, as it does not require a fish host to survive. It is an opportunistic parasite that strikes when a fish’s immune system has been compromised. Uronema mainly affects damsels (especially chromis) and clownfish.
  • Velvet (Amyloodinium) - 6 weeks
During the fallow period, the fish must be quarantined and treated for whatever disease(s) are afflicting them (see links below). Successful treatment is imperative to avoid disease(s) from being reintroduced into the DT:

Fish Diseases

Medications and Treatments

Quarantining all future livestock purchases is also very important to avoid having to go fallow again in the future: How to Quarantine

After the fallow period has ended, it is a good idea to test your DT with 2-3 freshwater black mollies to ensure no diseases are still present. A freshwater black molly will have no immunity to marine diseases, and evidence of ectoparasites (white spots) or flukes (translucent spots) are easier to see on a dark coloured fish. More information here: Keeping, Breeding, and Raising Saltwater Mollies

Once you are ready to start returning your fish to the DT; I recommend doing it a few fish at a time, spread out over a couple of weeks. This will give your bacterial levels time to adjust to the added fish bio-load, and avoid a potential mini-cycle/ammonia spike. I also prefer to add back smaller fish first, so they are established ahead of the larger, more aggressive ones.

Do be sure to closely monitor your ammonia levels while adding fish back. I advocate using a Seachem Ammonia Alert badge for constant monitoring:



Dr. Fish
Why a fallow period will sometimes fail

We've all seen claims of ich, velvet, etc. returning after a 76 day fallow period.

Oftentimes, a fallow period failure is due to human error: The sick fish weren't treated long enough or the treatment itself wasn't done properly, cross contamination via wet hands or equipment, aerosol transmission (more info). It is also possible that undiscovered strains of ich (and other diseases) exist; ones with a prolonged life cycle that exceeds what we know to be true from scientific research. However, there is also this possibility to consider:

Dormancy induced by a hypoxic environment in tomonts of Cryptocaryon irritans, a parasitic ciliate of marine teleosts
We incubated tomonts of Cryptocaryon irritans in a hypoxic seawater environment (1.4–1.7 mg/L O2) (low dissolved oxygen; DO) and examined their development using a acetocarmine whole-mount staining method we developed for nuclear staining. They showed little development and stayed in the dormant phase in the hypoxic environment. When transferred into the hypoxic environment after incubation in an oxic environment (air-saturated, 8.7–8.9 mg/L O2) for 1–4 days, their development stopped in 1 day. However, when dormant tomonts generated in the hypoxic environment were transferred to the oxic environment, they resumed development and released theronts. These results indicate that tomonts can become dormant when exposed to a hypoxic environment, but can resume development when exposed to an oxic environment at any developmental stage. When exposed to the oxic environment, tomonts recovered from 1-month dormancy and released as many theronts as control tomonts constantly incubated in the oxic environment. The infectivity of theronts from the recovered tomonts was similar to the control tomonts. Thermoclines prevent oxygen-rich surface seawater from reaching the bottom of water column and create a hypoxic sea floor environment in summer; these thermoclines are broken down in autumn or after typhoons. The long-term viability of dormant tomonts in hypoxic environments may be a key factor in the autumn outbreaks of cryptocaryoniasis in floating net cages in temperate waters.
Click to expand...
Highlights from the study:
  • This study demonstrates that tomonts of Cryptocaryon irritans become dormant in hypoxic environments.
  • Dormant tomonts resume development in oxic environments at any developmental stages.
  • We examined tomont viability following variable sequences of oxic and hypoxic conditions.
  • Dormancy in hypoxic environments may be key to the autumn outbreaks of cryptocaryoniasis in floating net cages in temperate waters.
So what does this mean for us and our fallow aquariums? Primarily, the study showed that an ich tomont (the "egg stage" which encysts to corals, inverts, rocks, etc.) can go dormant if the protomont crawls into a hypoxic (low oxygen) environment or anaerobic (no oxygen) region of your DT just before encysting. Examples of this include under your sand bed (especially a DSB), inside a non-porous rock, any "no flow" region of a canister or other aquarium filter. The study also demonstrated that once returned to an oxygen rich environment, these once dormant tomonts resumed their development and released theronts (free swimmers which seek out fish to infect.) How long can it take for a dormant tomont in a hypoxic environment to suddenly be exposed to an oxic (oxygen rich) environment? The world may never know?! :eek:

So what can you do to eliminate low oxygen areas of your DT during a fallow period?
  1. Take any canister or enclosed filters offline, and sterilize them with bleach. Without fish to foul the water, your DT will be fine with just rock/sand for filtration and good water circulation.
  2. Speaking of circulation, crank up those pumps for maximum flow & gas exchange throughout the aquarium. (Don't forget to add a pump down in the sump.)
  3. Blow out your rocks (using a powerhead) and vacuum the sand during water changes whilst going fallow. This will "stir things up" and provide free oxygen to those areas.
How can I setup my Display Tank to be "hypoxic proof" just in case I ever have to go fallow?
  1. Only use filtration with an open top (like a sump), and avoid canister filters and other filters which may contain anaerobic regions. If needed, take these offline if ever having to go fallow.
  2. Use just a light layer of sand; the deeper it is the more likely tomonts can get "trapped" down under there.
  3. Never have sand out of reach (i.e. under a rock) in case you need to vacuum it during a fallow period.
  4. Only use very porous rock which will allow plenty of flow (and oxygen) to pass through.
More information on Marine Ich (Cryptocaryon irritans) can be found here: https://humble.fish/community/index.php?threads/marine-ich.11/