Short answer for an elderly white cloud
An elderly white cloud mountain minnow should be checked over visually once a week, while weighing should be rare and only monthly at most. This is a small, slender, cool-water shoaling fish, and repeated netting for a scale reading can cost more in stress than the number gives back. The weekly look, done while the shoal feeds, tells you most of what a weight would: body fullness, fin colour, breathing, and whether this minnow still keeps up with the group.
Why this minnow is hard to judge by weight
A white cloud mountain minnow carries very little body mass, and an older fish naturally looks narrower than a young adult. A full belly after feeding, eggs in a female, or a single missed meal can swing the appearance of such a small fish within hours. Handling also risks damaging the delicate fins that show this species' red and gold edging. For an elderly white cloud, a consistent side-on photograph beside the same plant or stone each week is a kinder and often clearer measure than a scale, because it shows the back line and belly shape changing over time.
The weekly check at feeding time
Watch the elderly white cloud when food goes in, because feeding reveals appetite and strength together. Note whether he rises with the shoal or hangs back in the plants, whether he takes small foods easily, and whether faster tankmates finish everything before he gets a share. Look for a belly that stays hollow long after feeding, fins that look ragged rather than neatly edged, and a back that looks sharply ridged from above. Check his breathing while he rests, since a minnow working hard to breathe at rest needs attention even if he still eats. Compare this week's note with last week's rather than judging from memory, because a white cloud fades gradually and the human eye adjusts to slow change until the fish suddenly looks very different from photographs taken a month apart.
If you do weigh this fish
If a vet has asked for weights, weigh an elderly white cloud mountain minnow no more than once a month and always in water. Scoop him gently in a small container of tank water rather than lifting him in a net through the air, zero the scale with the container first, and keep the whole process under a minute. Do the weighing on the same day pattern each time, before feeding, so readings compare fairly. Stop the routine if he becomes hard to catch, because a long chase through the tank defeats the purpose for an old fish.
Changes that should not wait a week
Seek aquatic veterinary advice quickly if this minnow leaves the shoal entirely, stops feeding, loses the red edge to his fins alongside rapid weight loss, or rests on the bottom gasping. In an elderly white cloud, hiding alone is a marked change because this species normally stays loosely with its group even when resting. Test the water when you notice the change, and take the recent weekly notes with you. Storing those dated observations in KinsLe's Pet Passport lets a vet see this minnow's slow trend instead of relying on a single difficult weighing.