This live culture contains over 300 Daphnia magna. Please see pictures and videos to see exactly what a 300+ count culture looks like.  
Daphnia magna are highly nutritious, tiny crustaceans that multiply in large numbers quickly. They are an exceptional live fish food known for being the “go to” for conditioning breeding fish.
These daphnia can be given immediately to your fish as live food or can be used as a starter culture to start your our thriving colony. 
They can be kept in many ways but do best when raised in a dedicated container by themselves. Larger water volume is better, but 10-20 gallons is a good place to start if you're hurt for space. Also, the container doesn't need to be a glass aquarium.  
I like using an air line with no air stone for some water movement. I'll use a few 6mm stainless steel nuts to weigh down the floating airline. You want large air bubbles and low flow.
Daphnia reproduce extremely fast with adults producing many dozens of eggs every 5-7 days and those babies maturing in a week or less. This explosive growth is exciting to see but presents a challenge when judging how much to feed. The issue is that as time goes on, your cultures population will grow exponentially. You tend to feed more and more as you see the population booming. This can quickly lower the water quality as daphnia are filter feeders and only consume the food particles in the water column. The food quickly settles to the bottom and rots, creating water issues. This feeding challenge can be avoided by feeding with live green water.
Without a doubt, the best food choice for daphnia is live green water. If live green water is unavailable, many have had success feeding dried chlorella algae and yeast. You can mix a teaspoon of powder algae with a glass of water and feed every few days. Feed just enough to cloud up the water. As they eat, the water will clear up. That's when you can add some more. 
It's a good idea to have several daphnia cultures going as they are prone to crashing due to explosive growth and water quality issues from feeding them too much.
When setting up a new culture or doing water changes, it would be best if possible to use aged aquarium water. Next best might be bottled water or filtered water. There is lots of conflicting information on the internet about daphnia being sensitive to water conditioners. More experimentation is needed to determine if there is validity to those statements.