One of the greatest attributes of the Medaka is its ability to overwinter outdoors in climates that would quickly decimate tropical species. However, ensuring their survival requires careful preparation before the first freeze sets in.
Metabolic Dormancy
As water temperatures dip below 50°F (10°C), Medaka will naturally enter a state of dormancy. They will travel to the bottom of the pond or tub, moving infrequently to conserve energy. Crucially, you must stop feeding them. Once metabolic rates plummet, any food ingested can rot in their digestive tract, causing fatal complications.
Preparing the Tub
The goal is not to heat the tub, but to prevent it from freezing solid.
- Depth is Key: Ensure your outdoor tubs are at least 12-16 inches deep. The surface may ice over entirely, but the deeper water will remain above freezing due to geothermal insulation.
- Clean the Sludge: Remove excessive detritus and dying plant matter before dormancy begins. Decomposing organic matter under ice will deplete the oxygen and cause toxic gas build-up.
- Break the Surface Tension: Use an airstone or a floating de-icer to keep a small hole open in the ice layer. This is vital for gas exchange during prolonged deep freezes.
Curator's Note
Leave hardy floating plants like water hyacinth or thick layers of hornwort in the tub. They die back but provide a structural matrix for Medaka to nestle into at the bottom, sheltering them from biting currents.