The first 30 days of a Medaka’s life are the most critical. This is where phenotypic expression begins to lock in, fueled primarily by environment and nutrition. If you want robust adults, zero corners can be cut during the fry stage.
Water Parameters & Temperature
Stability is paramount. While adult Medaka are exceptionally hardy and can withstand vast temperature swings, fry are delicate. Aim for a consistent 75°F - 80°F (24°C - 27°C). Use mature, green water if your setup permits—it provides an ambient layer of infusoria for them to graze on constantly.
The Infusoria Paradigm
Powdered fry foods are great secondary sources, but live infusoria (microscopic organisms) remains unbeatable for the first week. Newly hatched fry will lack the mouth gape for baby brine shrimp or microworms initially. Introduce carefully cultured paramecia, and watch their growth rates double.
Curator's Note
Overfeeding fine powders fouls the water faster than fry can consume it. Siphon the bottom of the rearing tub lightly every second day to prevent catastrophic ammonia spikes. Perform minimal 10-15% water changes using water of identical temperature.
Transitioning Foods
By day 10, the largest fry should be ready to accept freshly hatched baby brine shrimp (BBS) and micro-pellets. This is the stage where their colors will begin to slightly differentiate. Culled selection shouldn't happen yet—allow them at least 45 days to begin showing their true lineage traits.