Nothing in a reef tank photographs like a mandarin. That electric-blue, hand-painted pattern is the reason half the hobby's Instagram exists and it's also the reason so many mandarins quietly starve to death over two or three months in tanks that looked perfectly healthy on the surface.
Quick answer: Mandarin fish (genus Synchiropus) are strikingly colored but genuinely advanced-level fish. They're specialized micro-predators that graze on copepods all day, so they need a mature tank ideally 9-12 months old, 30-55+ gallons depending on species with a well-established pod population, not just live rock and good intentions. Captive-bred mandarins, now widely available, are a far more forgiving starting point than wild-caught ones.
Meet the mandarins
"Mandarin fish," "mandarin goby," and "mandarin dragonet" all refer to the same small family of fish in the genus Synchiropus they're dragonets, not true gobies, despite the common name. Three color forms show up regularly in the trade:
Green mandarin (Synchiropus splendidus) is the one everyone means by default a bright blue body threaded with swirling orange stripes and a blue-green face. It's the most widely available and the most photographed reef fish there is.
Spotted mandarin (Synchiropus picturatus), also called the psychedelic mandarin, is light gray-green covered in black, pink, and blue spots rather than stripes. It's slightly smaller and often tolerates a slightly smaller tank than the green mandarin, though the care demands are otherwise nearly identical.
Ruby red and other color morphs occasionally turn up as line-bred variants of the green mandarin, but they carry the exact same husbandry requirements the color is cosmetic, the care level isn't.
Why the care level is "advanced," not "easy"
Mandarins are specialized micro-predators that evolved to graze on tiny crustaceans continuously, all day, every day. They don't have a stomach in the way most fish do, so they can't gorge on one big feeding and coast they need something to pick at almost constantly. That's the entire reason this fish has a reputation for being hard: it's not disease-prone or fragile in shipping, it's just wired to eat in a way a typical aquarium doesn't naturally support.
How big a tank does a mandarin fish need?
Green mandarins do best in a well-established tank of 55 gallons or more, with roughly 100 pounds of live rock per fish to support a large enough copepod population to sustain them. Spotted mandarins can get by in something closer to 30 gallons, but both species need the tank itself to be mature nine to twelve months old at minimum before the copepod population is dense enough to feed one.
Do mandarin fish only eat copepods?
Wild-caught mandarins largely do, at least at first, which is exactly why so many starve in newer tanks that simply haven't built up enough pods yet. Captive-bred mandarins are the better answer here: thanks to sustainable aquaculture, many are now raised already trained onto frozen mysis and prepared foods, so they'll actively hunt live pods between feedings rather than depending on them entirely. If you're set on keeping one, starting with a captive-bred specimen is the single biggest thing you can do to stack the odds in the fish's favor.
Keeping the pod population alive alongside the fish
A mandarin's survival really comes down to whether your tank can keep producing copepods faster than the fish eats them.
Supplement with live pods weekly for the first two to three months after adding a mandarin, then taper to monthly once the population looks stable and visible on the glass and rockwork after lights-out.
Avoid stocking heavy pod-competitors alongside a mandarin scooter blennies and other dragonets have identical dietary needs, and many wrasses and some dottybacks graze pods hard enough to starve out a mandarin sharing the tank.
Give the tank time before adding the fish at all. A six-month-old tank is the bare minimum; nine to twelve months lets the pod population actually establish itself rather than getting wiped out in the mandarin's first few weeks.