The direct answer for an ageing guppy
An ageing guppy eats more easily when every piece of food is small, soft, and waiting in calm water it can reach without a sprint. A senior guppy's mouth and jaw strength are not what they were, and its tail, often carrying years of finnage, pushes less water than it once did. Big flakes that a young guppy shreds happily become a struggle. Crushing food to a fine crumb, feeding it over a sheltered patch of surface, and splitting the day's food into two smaller offerings turns mealtime back into something this fish can finish rather than watch drift away.
Why senior guppies lose out at the surface
Guppies feed at the top, and the top of a community tank is contested ground. An older male with a long, heavy tail swims like someone dragging a parachute; by the time he climbs to the feeding ring, faster tankmates have taken the best of the scatter. Ageing females, often larger, may hold their own for food but tire quickly and drop back to the plants with less than they need. A stiffening spine, common in old guppies, also makes the quick upward dart for a sinking crumb harder, so food that falls past the first few seconds is effectively food this fish never sees.
Building a calm feeding corner for this guppy
Choose the surface corner farthest from the filter outflow and make it this guppy's dining room. A floating feeding ring or a simple circle of plant cover holds the crumb in one place instead of letting it sheet across the tank. If your filter allows it, reduce or pause the flow for the two minutes of feeding so the surface stills. Sprinkle the crushed food inside the ring only when the senior guppy is already nearby, and feed the rest of the tank at the opposite end a moment earlier. That small head start, repeated daily, is usually the difference between a guppy that eats and one that merely attends meals.
Size, softness, and frequency that suit an old guppy
Think crumb, not flake: pieces should be small enough for this fish to take whole, because an ageing guppy cannot tear at oversized food the way it once did. A short soak in tank water softens dry food for a weaker bite. Two or three modest feeds suit a senior metabolism better than one feast, and they spread the chance to eat across the day if one session is missed. Rotate in finely chopped frozen foods, thawed fully, for variety an old fish will still make an effort for. Remove whatever drifts untouched after a few minutes so yesterday's generosity does not foul the water this fish breathes.
The point where appetite loss needs a vet
An ageing guppy that stops rising to the feeding corner, even when food lands beside it, needs a vet rather than another menu change. So does a fish whose belly swells while its back thins, one that rests on the substrate between efforts, or one showing clamped fins and a rocking, shimmying swim. Note when the change began and whether tankmates are still eating normally, because a single fading fish tells a different story from a whole tank off its food. Keeping those dated appetite notes in KinsLe's Pet Passport gives the vet this guppy's whole recent pattern in one portable record when you arrive.