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To answer your question, The larger your volume of water the more daphnia you will be able to raise. Also a larger volume is easier to keep stabilized and the chance of fouling the culture through overfeeding is minimized. I currently have 7 indoor cultures ranging in size from 3 gallons up to 20 gallons. I will be inoculating a 300 gallon outdoor stock tank in the next couple of weeks. I have not noticed any significant differences in reproductive or growth rate between different volumes of the cultures.
Having basically said....larger is better...I don't want to dissuade any readers from trying to raise daphnia in small containers. Case in point is my successful raising of daphnia in a 3 gallon container for over a year.
The following photos show a 3 gallon plastic "cheese balls" container. My son gets these from Costco I believe and he gave me one about a year ago. The second photo is of the culture this week before siphoning off 1 gallon of solution and the last photo shows the amount of daphnia that was contained in that one gallon. As you can see that is more than enough daphnia to feed several tanks. I would suggest if you were to try this that you would use a "turkey" baster" to feed some of the daphnia and put the rest back into the culture.
So..yes..a larger tank to raise daphnia would increase the number and lesson to possibility of a crash but you can also raise them in smaller containers.