Bought a Mandarin dragonet, as well as some copepods to put in the tank. Will it survive on these alone or must I continue to add more on occasion or buy some other food? How do I know if the copepods are alive & well? Have no experience with these creatures.
You'll be able to see the copepods moving around on the rocks and throughout the tank. Dont look in the water column, they dont just freely swim around, look on the rocks and on the sand.
Depending on how much live rock you have and how many fish are eating the copepods, you may or may not have to buy more. A well established aquarium of 90g can support one mandarin as long as you have enough live rock or other breeding grounds for the copepods.
Of course, if you only put in 10 and the mandarin wipes them out before they have a chance to reproduce, you'll have to buy more.
Read this: Mandarin Primer. By the time you have finished the thread and associated links, you will be "armed and dangerous" with Mandarin knowledge. Hopefully the little one will benefit from that and survive too.
If you didn't see a healthy population of copepods in your tank before you added the mandarin, then unfortunately, it is not likely to survive. A refugium, or even a large ball of chaetomorpha in the DT can help establish a healthy pod population.