The direct answer for an ageing zebra danio
An ageing zebra danio needs the surface slowed down. Young danios are streaks of silver that treat feeding as a race and win it; an old danio still has the instinct but not the acceleration, and food drifting on a brisk current is gone before it crosses the tank. Confining fine crumbled food inside a floating ring, in the corner where the filter's push is weakest, turns the race into a meal this fish can actually join. Small pinches, repeated, suit a senior danio far better than one generous scatter that the youngest fish in the tank convert into a ten-second memory.
Why an old danio misses food it clearly wants
Watch an ageing zebra danio at feeding time and the problem is visible: it aims correctly, starts promptly, and arrives late. Muscle and reaction both dull with age in this species, and a lifetime of hard swimming can leave an old fish with a slightly stiff, less fluid stroke. Surface flow compounds it, carrying crumbs sideways faster than the senior can track. Tankmates bred from the same restless mould show no mercy, working the surface in tight loops. The ageing danio eats the leavings at the edges, which is why these fish often slim down while their behaviour still looks bright and willing.
A ring, a calm corner, and a head start
Anchor a floating feeding ring in the surface corner most sheltered from the outflow, ideally near plants where this danio already rests between patrols. Crumble the food finely before it goes in; pieces small enough to be taken in one snap suit a mouth that no longer tears at larger flakes. Feed this corner first, before the main scatter for the rest of the tank, so the senior danio is feeding while its rivals are still travelling. If your filter can be paused briefly, those two still minutes are worth more to this fish than any special food. Consistency matters as well: the same corner, the same ring, the same order of events lets an old fish pre-position itself by habit.
Portions tuned to a senior danio's day
An ageing zebra danio's digestion, like its swimming, does better with little and often. Two or three small ring-feeds are the pattern to aim for, each cleared within a couple of minutes, rather than a single heavy feeding that overwhelms the fish and then fouls the water it lives in. Rotate in fine frozen foods, fully thawed, on some days; the stronger scent draws a senior fish to the ring promptly. Skim out anything uneaten once the school drifts off, because decaying surface food films the very layer this top-dwelling species spends its life in. Weigh the routine weekly by eye: a healthy old danio keeps a gentle plumpness along the belly between the stripes.
When slowness is a symptom, not just age
A vet should see this danio if it stops coming to the ring it knows, because abandoning a learned feeding spot signals more than stiffness. A bent or humped spine that worsens, persistent loss of condition despite a protected share of food, or a fish that separates from the school and hovers in plants all day are reasons to book in. Note whether the whole school is thriving, since that contrast helps a vet judge an individual decline. Keeping a brief record of this danio's feeding success in KinsLe's Pet Passport means its gradual change is documented, not guessed at, when you describe it.