We've all seen claims of ich, velvet, etc. returning after a 76 day fallow period. (For anyone wondering what a fallow period means click here: https://www.reef2reef.com/threads/fallow-periods-going-fishless.190324/)
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
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 can you do to eliminate low oxygen areas of your DT during a fallow period?
- 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.
- 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.)
- 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.
- 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.
- Use just a light layer of sand; the deeper it is the more likely tomonts can get "trapped" down under there.
- Never have sand out of reach (i.e. under a rock) in case you need to vacuum it during a fallow period.
- Only use very porous rock which will allow plenty of flow (and oxygen) to pass through.