Give the school a sheltered home base
An elderly harlequin rasbora has spent its life moving with the school, and old age does not change the instinct, only the stamina. Daily life improves when the school has a planted end of the tank to call home: dense stems and broad leaves at one side, open water at the other, so the group can drift into cover together instead of scattering. Keep that planted end permanent. Harlequins orient themselves by familiar structure, and an elderly fish that finds its landmarks moved after every cleaning wastes the little energy it should spend on feeding.
Quiet the current and the light
Harlequin rasboras come from slow, shaded forest water, and an elderly one feels excess flow and glare more keenly than the youngsters do. Angle the filter return into the glass or fit a baffle until the planted corner moves only gently, and float plants above it to break the light. Under calmer, dimmer conditions a senior harlequin holds its copper colour and stays with the group; under glare and push it fades, hangs at the back, and misses meals. These are cheap, reversible changes, and they usually show their benefit within days.
Feed the corner, not the whole tank
Feeding an elderly harlequin rasbora is about geography: it eats where the school rests, in the middle of the water column, taking small items as they drift down. Release fine food just upstream of the planted corner so it sinks slowly through the group, and add a second tiny pinch directly over the old fish when it hangs slightly behind the others. One large surface scatter favours the fastest fish and teaches the slowest to stop bothering. Two small, placed feeds a day keep a senior rasbora's intake honest without overfeeding the tank.
Keep the group whole and the tank peaceful
An elderly harlequin rasbora alone is a stressed rasbora, so protect the school's numbers and temperament. If the group has thinned over the years, add a few healthy, fully grown harlequins of similar size rather than juveniles that dart and bicker, and quarantine them first. Rehome anything that nips or monopolises the middle water. Skip other late changes — no new lighting rig, no rescape, no novel foods introduced all at once. For a senior schooling fish, sameness is not boredom; it is the condition under which it feeds, rests, and stays coloured up.
Notes that keep the gentle routine intact
The adjustments behind an elderly harlequin rasbora's comfort — its planted corner, the baffled corner flow, the twice-daily placed feeds — take two minutes to hand over and are lost the moment they are only spoken. Record them with brief notes on appetite and colour, the two quickest daily reads on this species' health. Keeping that record in a KinsLe Pet Passport puts this rasbora's care details into one portable packet, so a holiday feeder or aquatic adviser can hold the routine exactly as the fish knows it.