Begin at the opposite end from his resting place
A bottom-resting elderly fish has usually claimed one favourite patch of gravel, often in the quietest corner away from the filter outflow, and he returns to it after every disturbance. Start your vacuuming at the far end of the tank from that patch. He then experiences the gravel stirring, the vibration through the glass and the falling water level at a distance first, while he is still lying somewhere he trusts. Work along the tank in steady strips so the cleaned area grows toward him predictably. By the time you are near his corner he has had several minutes to judge that nothing is chasing him, which is exactly the information an old, slow fish needs before he decides whether to move at all.
Keep the tube slow, low and predictable
Push the siphon tube down into the gravel, let the debris lift and swirl inside it, then draw it out smoothly before moving a tube's width along. Jabbing the tube in and out sends thumps through the water that a resting fish feels through his whole body, so keep every movement unhurried and rhythmic. Keep your free hand out of the tank entirely; a hand appearing beside a fish that can no longer dart away reads as a predator arriving, not as housekeeping. If your bucket sits well below the tank, the siphon flow stays steady, and a steady flow matters here because repeatedly starting and stopping the suction near him turns one disturbance into many.
The patch he lies on gets one gentle pass
When you finally reach his resting patch, do not shoo him off it and do not chase him around the tank so you can clean underneath him. Raise the tube slightly so the intake hovers just above the gravel surface: loose mulm and uneaten food still draw up, but the pull on his fins and belly is far weaker. If he chooses to shift aside of his own accord, vacuum the patch properly while he is off it — then stop, even if the corner is not spotless. An elderly fish's comfort sets the standard for this job, and a slightly grubby corner is a fair price for a fish that never had to flee his own bed.
Read his breathing while you work
A resting old fish breathes with slow, even gill movements. If those movements speed up, his fins clamp or he makes a sudden clumsy dart, he is telling you the session has become too much: rest the tube against the glass away from him, stand still, and give him a minute to settle before deciding whether to finish or call it done. Vacuuming half the tank per session suits him anyway, because it keeps each water change smaller — and smaller changes mean gentler swings in temperature and chemistry for a fish with little reserve left for shocks.
Keeping the gentle routine consistent
This slower pattern only protects him if it survives a change of hands. KinsLe's Care Teams keeps the vacuuming rota, the note about which corner is his resting place and the gentler hovering technique together on his profile, so the next person who maintains the tank works around him in exactly the same way.