Short answer for an elderly guppy
An elderly guppy should get a calm visual check once a week and, if weighing is not stressful, a brief weight check about once a month. A guppy is so small and light that handling itself can do more harm than the number is worth, so the weekly look matters more than the scale. During that look, judge the plumpness behind the head, the fullness of the belly between meals, and whether the tail still fans out instead of drooping.
What changes in an old guppy
An elderly guppy often loses muscle along the tail stalk first, so the body looks front-heavy even while he still comes up for food. The spine may curve a little more, colours fade, and the flowing fins of a male can fray or stay partly clamped. He may rest more on broad leaves, hover near the surface, and lose the quick darting movement he had when younger. None of those signs alone proves illness in an old guppy, which is why a dated weekly picture from the same angle is so useful for seeing the direction of change.
How to weigh a guppy without a chase
Weighing an elderly guppy should be brief and fully in water. Fill a small clear container with tank water, place it on the scale, and zero the reading before guiding the guppy in with a soft net or cup. Keep the container shaded, read the weight once, and slide him straight back into the tank. Skip the weighing entirely if he panics, if the water is due a change that day, or if he has just eaten, because a full stomach makes a tiny fish's reading misleading. If netting is always a struggle, stop weighing and rely on weekly side-on photos and body shape instead.
The weekly hands-off check for this fish
A weekly check for an elderly guppy takes a few quiet minutes at feeding time. Watch whether he reaches the food as quickly as his tankmates, whether he eats or only mouths flakes, and whether younger fish push him aside. Look along his back for thinning, check the belly from above for a pinched look, and note any white edges, spots, or tears on the fins. Check that he can hold himself level rather than tilting, and that he is not gasping at the surface between feeds. Write down appetite, swimming, and fin condition in plain words, not just a weight.
When an elderly guppy needs a vet sooner
Contact an aquatic vet promptly if this guppy stops eating for more than a day, loses visible body mass over two weekly checks, or develops rapid breathing, sores, or a badly curved spine that appears suddenly. A sudden change in an elderly guppy is different from slow fading, and water tests should be checked at the same time because poor water often explains a fast decline. Bring the recent notes and photos to the appointment so the vet can see how quickly the change happened. Keeping the weekly observations and any monthly weights in KinsLe's Pet Passport gives that guppy a clear record to hand over at the visit.