Why did my daphnia (moina) culture crash? 7 causes and fixes

Every culture keeper knows the scene: the tub was cloudy with animals on Friday, and on Monday there is clear water, a sad film on the bottom, and nothing moving. Moina and daphnia cultures rarely die for mysterious reasons — they die for one of seven ordinary ones, most of which announce themselves a day or two early if you know what you are looking at. Work through this list before you blame the starter you bought.

1–3: Food, food and water

• Overfeeding (the champion). Uneaten yeast or spirulina rots, bacteria bloom, oxygen crashes, animals die — in that order. Fix: feed only when the water has nearly cleared; a faint haze means food is still present. A hungry culture recovers; a fouled one does not.
• Starvation after a boom. A culture that exploded last week has ten times the mouths today. If the water is gin-clear and animals are thinning, feed — little, twice a day.
• A big water change with the wrong water. Fresh chlorinated tap water, or water much colder than the tub, can wipe a culture overnight. Fix: aged or dechlorinated water, same temperature, partial changes.

4–5: Temperature, both directions

Heat first, because it is the South African killer: moina cultures cook in a sunlit garage or a tub against a warm wall in summer. Sustained heat plus the bacterial bloom it encourages ends cultures in hours. Move the tub to the coolest room, shade it, and float a frozen bottle in real emergencies.

Cold is the slower version: below ±20 °C moina reproduction stalls, the culture looks “dead” for weeks, and keepers throw out a tub that would have boomed again in spring. Warm it up before you write it off.

6–7: Harvesting, age — and the missing second tub

A culture that is never harvested ages: older animals, accumulating waste, declining births. Harvesting a third of the tub twice a week is not taking from the culture; it is what keeps it young. Conversely, harvesting a brand-new culture to the last animal in week one ends it before it starts — let it build first.

And the cause under all the others: one tub. Every experienced keeper runs two cultures because crashes are a when, not an if. Splitting a thriving tub costs a bucket and five minutes; restarting from nothing costs you a spawn. If your culture has crashed, reseed from the backup — and if you never had one, that is the fix to make today.