The short answer: a small rigid pot, kept dark
A small rigid container with a secure lid suits a celestial pearl danio best for a short trip to a specialist. These are some of the smallest fish in the hobby, shy, cool-water, and easily crushed by casual handling, and that combination tilts the choice away from a standard fish bag. A bag can be squeezed in a car door moment, pressed under other items, or fumbled at the practice desk. A sturdy specimen pot or food-grade tub keeps its shape through all of that, holds a steadier temperature for its size, and opens at the specialist's bench without anyone having to net a fish the size of a fingernail out of floppy plastic.
Never send one danio alone
Taking several schoolmates along is part of the right container choice for a celestial pearl danio. A lone danio in a pot is a fish spending the whole trip in a state of alarm, clamped and colourless, which also makes the specialist's job harder because a terrified fish shows nothing like its normal behaviour or colouring. A small group settles together, colours up sooner, and lets the specialist compare the fish you are worried about against its healthy companions, which is often the fastest route to useful advice. Fill one generous pot for the group rather than splitting them across several, and add a sprig of plant from their tank so the fish have something to weave through and hide behind.
Filling the pot with cool home water
Water from the danios' own tank is the fill this container needs, because celestial pearl danios live cooler than most tropical fish and notice abrupt changes in small volumes quickly. Fill the pot about two-thirds full so there is air above the water, close the lid firmly, and wrap the pot in a dark cloth or set it inside a small insulated box. That darkness is not a nicety for this species: a danio that cannot see hands and headlights settles within minutes instead of dashing against the walls. Catch the fish with a cup or the gentlest fine net, and do not feed them on the morning of the trip, so the pot's water stays clean through the appointment and the drive home.
During the appointment
Keeping the pot closed and shaded while you wait protects celestial pearl danios better than showing them to anyone in the room. Tell the specialist the water in the pot is the group's home water and roughly how cool their tank runs, because a well-meaning transfer into warm water is a real risk for a species whose comfort range sits below standard tropical. If the specialist needs to keep the fish for a while, the plant sprig travelling with them gives the group immediate cover in any holding tank. Ask before leaving whether they should travel home in the same water or fresh prepared water, particularly after a long wait, and follow that advice rather than deciding in the car park.
Settling the group back home
A returning group of celestial pearl danios should be acclimated by floating their pot or standing it in the tank's water until temperatures match, then mixing tank water in gradually before release. Keep the tank lights low for the first evening and let the group slip back into the planted areas they know. Offer a small feed once they are moving through the plants as a group again. Individuals that remain clamped, pale, and separate from the school into the following day, or that stop feeding, are worth reporting back to the specialist promptly. Noting the group's usual temperature, planting, and feeding routine in a KinsLe Pet Passport ensures the specialist sees these danios as the cool-water school they are, not as generic tropical fish.