I have a Green Mandarin (Synchiropus splendidus) in my nearly 3 year old 37 g mixed reef tank... lots of LR but who knows how many (or how few) copepods are crawling on the rocks? After trying several different types of foods, I found that the Mandarin loves small, frozen mysis shrimp. Think twice before you consider buying copepods (see below).
There are several brands of frozen mysis shrimp. Not all brands will work, because some provide shrimp that are too large for a Mandarin's small mouth. Some brands offer very small and fairly uniform sized mysis. If you try one of those - such as the ones sold under the Hikari brand, you might have as much success as I've had. These 1/4 inch or smaller size shrimp are white (just like copepods) and are readily accepted and relished by my Mandarin.
You may be able to find even smaller ones. Do an internet search for both Mysis Shrimp and Sea Horses. Many folks successfully feed Sea Horses small Mysis shrimp - and if they are small enough for Sea Horses, they are small enough for Mandarins - although that does not guarantee that the Mandarin will eat them.
You can buy copepods from several reliable internet sites, but unless you want to raise them, you'll spend $20 to $30 a pop (plus shipping) each time you feed your Mandarin - not too cool. I bought over $100 worth of those little nothings - 4 different sizes, including "Tiger Pods" - and raised them in a 5 g pod breeding tank. Remember, copepods are tiny - just barley large enough to see with the naked eye. A Mandarin must consume hundreds of pods each day to sustain itself... even "Tiger Pods" are tiny.
If you are willing to raise pods (an easy thing to do btw... although it eventually becomes a chore), keep in mind that it takes 3 - 4 weeks before they reproduce in sufficient numbers to take a bunch from a 5 or 10 gallon breeding container and put them in the main tank.
As for relying on a small refugium to yield sufficient pod populations, I am convinced that the concept sounds better than it works. Ok... OK... if you have a 40 gallon fuge with lots of LR in it, you'll have a lot of pods growing in the fuge... but we are talking about small systems here.
I dumped a couple of bottles (one of Tiger Pods and one of three miniscule varieties of copepods) into my 10 g refugium. Can I see any pods in the fuge? On occasion I see a few... but large numbers of pods in the fuge., there ain't.
It's far easier to feed a mandarin tiny, frozen mysis shrimp.
