To me it is Joe, for this purpose. Unlike Sophia, I could not keep daphnia alive in buckets or anything smaller than the old bath I use, or a fibreglass pond. That is all I meant - I had more success in containers with larger surface areas. I believe daphnia use a lot of oxygen. It can obviously be done though, Sophia is proof! 8)
Just trying to come up with ideas of why you had all the sudden deaths.
Nobody sprayed anything nearby have they? Drift from neighbours?
Perhaps it was the milk. I have never added milk before but I would have thought, in direct sun, it would go off and perhaps pollute the water? I fed my daphnia green water, yeast, and/or dried blood.