Watch a few YouTube videos and read articles from Medaka fans, and you may come away thinking that these adorable fish can live in everything from ice to lava. Yet, Googling “best temperature for medaka” yields a smaller temperature range near 61-72°F (16-22°C).

To complicated matters further, “sudden” water temperature changes can shock the fish into a fatigues state.

Mentions of “sudden” water temperature changes typically refer to outdoor medaka-keeping in the fall/autumn season, and generally mention low water volumes as a contributing factor. In my case, a 60-gallon unheated aquarium in NY was too much for one of my California-sourced fish. Despite plenty of acclimation time, he began languishing at the bottom of the tank and had me running for the heater. Within a couple days of raising the temperature, he was happy as can be.