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Daphnia preservation

In order to maintain Daphnia in as dense a swarm as is sometimes found at a 
collecting site, it should be kept cool, and well fed. More Daphnia seems to 
die of starvation than of overcrowding or suffocation in a sense. There are 
several things to feed them. Commercial invertebrate foods for filter feeding 
marine animals work well if you avoid those products that are 99.9% distilled 
water. Tetra's Hermit Crab Meal also works. In the archives there are more 
Daphnia foods. However wild-collected Daphnia are likely to pulse, to all go 
dormant at once, and various other behaviors that stand them well in the wild 
but make them difficult to culture. 
  It's a step off-topic to suggest freezing them, but hey all food was once 
alive. Perhaps we could put all foods into 3 categories: not dead, recently 
dead, long dead. Rather than freezing pure Daphnia or "ice coating" it with a 
bit of water, I would suggest incorporating it into a gelatin diet or a 
Gordon's formula. Most of the good stuff in a Daphnia is going to leak out 
after thawing and cloud the tank rather than serve as food, so using it as an 
ingredient solves both problems.
   I have recently experimented with keeping some excess Daphnia magna in a 
cold refrigerator for the past week. It is still nearly all alive, moving 
very slowly. I've been told this works with baby brine shrimp and adult brine 
shrimp. Well, it works for Daphnia too, but how much longer than a week I 
have not yet found out.