The short answer
Start with thawed frozen foods offered in a calm spot at the same times each day, and make sure bolder tank mates are not stealing every bite. Give a new fish a few days to settle. If refusal continues and the fish looks thin, talk to an aquatic vet or an experienced marine shop promptly.
First, rule out the easy stuff
Many "picky" fish are simply new. A fish that arrived days ago may still be adjusting to your tank's flow, lighting, and pecking order, and appetite is often the last thing to return. Bold tank mates are the other classic culprit: a shy firefish or mandarin will not compete with a pushy tang at a group feeding, so watch one full feeding from start to finish and confirm the shy fish actually gets food. Strong current can also scatter meals before slow eaters reach them — try feeding in a sheltered corner, or briefly turning the pumps down.
Tricks that tempt reluctant eaters
Begin with what looks most like live prey: thawed mysis shrimp or enriched brine shrimp, offered small and often. Many keepers have luck soaking food in a little tank water with a drop of garlic-based attractant, or mixing a crushed bit of flake into frozen food so the fish starts to associate the new item with something familiar. Feed at the same times daily — fish learn routines quickly — and try target-feeding with a turkey baster so food lands right in front of the fish instead of drifting past. If the fish eats frozen food reliably, you can gradually blend in pellets over days or weeks. Never starve a fish into eating dry food; a weakened fish is harder to save than a picky one.
When picky becomes a problem
A healthy fish that skips meals for a couple of days while settling in is one thing. A fish that has not eaten for a week, is visibly thinning, hides constantly, or shows clamped fins or rapid breathing needs professional input. Do not guess at medications or treatments on your own — describe what you are seeing to an aquatic veterinarian, who can advise on sensible next steps. Keep a simple log of what you offered, when, and what the fish did with it; that timeline is genuinely useful to anyone helping you. KinsLe Messenger is your per-pet communications hub, so you can keep all those notes, questions, and feeding attempts for this fish in one dedicated thread instead of scattered across your phone. The KinsLe Messenger thread on KinsLe keeps your dated feeding log, what was offered, what got refused, and what finally worked, ready to show an aquatic vet.