Algaebarn.com -- Order copepods get next day shipping. Get 16oz per 20gallons of water. Go to LFS and buy Chaeto, or maidens hair algae or order it if no other choice. Place it in main display tank still wrapped up in the rubber band ball, maybe loosen it a bit. Get 1 bundle per 30g of water, should be size of softball.
Now place the algae in tank, loosen it a bit but no so much it grows wild. When copepods arrive, wait till night cycle, turn off powerheads and pump. Take the bag and start squeezing the pods into the tank into the chaeto or maidens hair clumps. Wait 1 hour and turn back on power heads and pump.
Order 16oz per 1 weeks for every 30g of tank water. Include the sump. Until you train them to eat non-live food. That is a whole other post and many posts explain training mandarins better than I can, as I follow their advice. For now you need to make sure they live long enough to be trained. Should only take 2-4 weeks to fully train them once you start. But don't be stingy with the pods for first two weeks, they are likely starved from the fish store.
My advice stops them from dying in the first two weeks, between now and then I suggest you research training them to eat frozen food, or decapsulated baby brine.
I also got one from the pet store 3 days ago. My tank had pods already, but I added more to fatten it up. I start training next week after he has had time to fatten up on pods in my tank. I will be using the glass bottle method, since I have a pretty active feeding frenzy tank.