Fallow Period Marine Tank 72 Days: Breaking Parasite Lifecycle

A fish-free display is the only way to clear a marine tank of ich without nuking the biology or moving livestock into copper. The fallow period marine tank 72 days starves Cryptocaryon irritans by removing every possible host for long enough that no encysted tomont can outlast the wait. This guide from Gensou Aquascaping in Singapore sets out why 72-76 days is the trusted duration, how to manage fish in a separate hospital tank while the display lies fallow, and what to check before reintroducing treated stock. Patience pays — a single shortcut can undo the whole effort.

Quick Facts

• Standard fallow duration: 72-76 days at 25-26°C display temperature
• Targets Cryptocaryon irritans tomonts, which can lie dormant up to 72 days
• All fish must be removed to a separate treatment system — no host in the display
• Corals, inverts, snails, and crabs can stay; they do not host marine ich
• Do not reduce the period even if the display looks “clean” at 50 days
• Temperature spikes above 28°C can shorten cycle but stress SPS corals; keep steady
• Post-fallow, only ich-free quarantined fish may be added — a single carrier resets the tank

Why 72 Days and Not 6 Weeks

Older hobby literature once suggested 4-6 weeks of fallow, based on faster tropical parasite cycles. More recent research on Cryptocaryon irritans tomonts, particularly at temperatures below 27°C, shows some encysted stages can remain viable for up to 72 days before releasing theronts. Going the full 72-76 days covers the tail end of that distribution. Cutting short at 6 weeks is a gamble that frequently fails in home reef tanks.

In Singapore, where ambient room temperature sits at 28-30°C, chiller-controlled reefs typically hold 25-26°C. That cooler end of the range is exactly where slower tomont hatching happens, so the 72-day minimum is appropriate.

Setting Up the Fallow Period

Net every fish into a separate quarantine or hospital system. Treat them there with copper, chloroquine, or TTM depending on diagnosis. The display stays running with full filtration, lights on a normal schedule, circulation unchanged, and feeding reduced to coral-only amounts (target-fed frozen mysis for LPS, amino acids for SPS). Cleanup crew stay put; they do not host marine ich.

Do not swap livestock, rocks, or water from another tank into the fallow system during the waiting period. Adding anything fishy resets the clock.

Managing Coral Health During Fallow

With no fish waste, nitrates and phosphates drift downward. Many reefers see dinoflagellate blooms or coral pale-out around week four. Dose amino acids, keep a small amount of protein skimmate dirty rather than ultra-clean, and consider supplementing with coral foods. Two-part dosing stays on the normal schedule; alkalinity burn is the bigger risk when nutrients drop.

Light schedule and flow stay unchanged. Skimmer output will decrease — that is normal, not a sign of a problem.

Monitoring and Record-Keeping

Log the fallow start date prominently on the sump or a phone calendar. Mark day 72 and day 76 as earliest and recommended end dates. Test ammonia weekly to catch any die-off. If a mantis shrimp or bristle worm kills a hidden wrasse mid-fallow, the cycle starts over from zero once the body is found.

Check every rock crevice with a torch before beginning fallow. A trapped fish carcass behind the rockwork can hide a still-feeding parasite host for weeks.

Treating Fish in the Hospital System

Parallel to the fallow, fish get proper treatment. Copper at therapeutic levels (0.35-0.5 ppm Cupramine) for 14-21 days, or the tank transfer method for 12 days, clears ich from the fish themselves. Add prazipro for flukes and metroplex if internal parasites or bacterial infection is suspected. Fish then move to a clean observation tank for the remainder of the 72 days.

Reintroducing Fish to the Display

On day 72-76, fish that passed both treatment and observation without symptoms can return. Acclimate slowly — salinity and alkalinity may have drifted slightly in the hospital system. Add them in the evening with lights off to reduce territorial stress from inverts and any remaining fish.

Any new fish added from this point forward goes through full quarantine first. The fallow has reset the display to ich-free; keeping it that way requires discipline on every future addition.

When Fallow Fails

The common failure modes are: a hidden fish survived the netting, a new fish was added too early, frozen food or live rock from an infected source was introduced, or the 72 days was cut short. If ich returns within weeks of reintroducing fish, the cause is almost always one of those four. Diagnose honestly before starting a second fallow.

Related Reading

Marine Ich Cryptocaryon Treatment
How to Set Up Marine Quarantine Tank
How to Quarantine Marine Fish Complete
How to Recover Crashed Reef Tank
Best Aquarium Copper Test Kit