A zebra danio that stops eating is breaking character
A zebra danio needs a vet when it refuses food for more than a day, because feeding eagerly at the surface is woven into everything this fish does. Zebra danios barely stop moving, chasing, displaying, and snapping up food in one continuous circuit of the tank's upper level. A danio that parks itself in a corner, ignores food landing beside it, and lets the school swirl on without it has made the most obvious appetite change a small fish can make. The contrast is your advantage: in a species this restless, you do not have to guess whether the behaviour is normal.
Why a zebra danio might still miss out
A zebra danio can underfeed through sheer social speed, because the school's feeding frenzy rewards the quickest fish and a weaker individual gets muscled off every flake. Long-finned varieties swim less efficiently than the standard fish and lose more of those contests. Danios also feed almost entirely near the top, so a fish that has been driven down by a territorial tankmate may get nothing even while looking active. Old age slows danios noticeably, and a senior fish eating smaller amounts gradually is a different picture from a prime adult stopping outright, which is why the speed of the change matters when you judge it.
Checking the individual inside the danio whirlwind
Check a zebra danio by feeding a little food at one end of the tank and watching the far end, where the less dominant fish feed once the school commits. Look for the fish that follows the group but does not snap, or that snaps and misses repeatedly. View the school from above during a pause, because a danio that is losing condition shows a narrower profile and, in some older fish, a curvature of the spine that is easier to see from overhead. Count the school too, since danios kept in small numbers feed and behave more anxiously than a proper group of six or more.
Zebra danio signs that call for a vet promptly
Call an aquatic vet when a zebra danio combines refusal with laboured breathing, a bent or wasted body, raised scales, or redness along the flanks. A danio resting on the bottom or wedged in plants, in a species that almost never rests in daylight, is an urgent picture. Several danios going off food together points to something shared in the water or the group, and that pattern deserves same-day advice rather than home experiments. Danios are small with fast metabolisms, so a fish that has eaten nothing for two days has spent reserves a larger fish would still be holding.
Handing the vet a zebra danio timeline
Hand the vet a timeline of the danio's change, noting when it left the school's feeding rush, whether it is a long-finned fish, and how large the school is. Include recent additions or losses in the group, your water test results, and whether the fish still moves normally between feedings or has slowed across the board. A top-down photo of the school helps the vet compare the affected fish's outline with its tankmates. Recording feedings, school changes, and water checks in KinsLe's Pet Passport keeps that danio timeline ready the moment you need to share it.