A schooling fish should not quarantine alone
Everything about a harlequin rasbora is built around the group, and quarantine has to respect that. A lone harlequin in a bare tank will fade to a washed-out silver, hug the bottom or a corner, and often refuse food, presenting a picture of sickness that is really loneliness and exposure. Quarantine your new harlequins together, the whole little school in one small tank, and they will settle within a day or two into the loose, easy shoaling that tells you they feel safe. A modest tank is fine for a school this small; what matters is company, cover and calm, not gallons.
Water and cover that let rasboras settle
Harlequins come from soft, slow, plant-choked waters, and a quarantine that echoes that world settles them fastest. Add a bunch of stem plants or a few silk plants to break up the open water, keep the light subdued for the first days, and run gentle filtration so the current does not exhaust them. Their native water is soft and slightly acidic, but do not chase those numbers in quarantine: stable water close to your main tank's readings is the right target, since these fish must soon live in your tank's water anyway. Warmth in the mid-seventies Fahrenheit and small, regular water changes keep conditions sweet for a group that eats small and wastes small.
Reading color and shoaling as health signals
A healthy harlequin advertises itself: rich copper-orange body, a crisp black triangular wedge, fins carried open, and a school that drifts together and tightens smoothly when a shadow passes. Against that baseline, the warnings stand out clearly. A fish whose wedge and color go pale and stay pale, fins clamped tight to the body, hanging alone away from the group, is telling you something is wrong with it rather than with its mood. Watch at feeding for the fish that rises but does not eat, or takes food and spits it repeatedly. Check skin at close range for the fine white grains of common spot diseases and for any shimmying, a side-to-side rocking in place that signals irritation or poor water.
Feed small, and feed like you mean it
Harlequins have small mouths and a preference for food in the water column, so offer a quality crushed flake or micro pellet in portions small enough to vanish in a minute, twice a day. Quarantine is your chance to learn that the whole school feeds with enthusiasm, and to spot the one individual that hangs back before it is lost among fifty tankmates later. Because a small quarantine with a whole school still loads its filter, test ammonia every couple of days during the first week and change water at the first hint of a reading. Fat, bright fish that have eaten well for a few weeks and shown no signs are ready to graduate.
Moving the school to the main tank
Float and drip-acclimate the group to the main tank's water as you would any fish, then release them together rather than in ones and twos, so no rasbora spends a frightened hour alone in a big new world. Add them with the lights low and let them find each other and the planted areas in peace. Expect the color to deepen further over the following days as they settle into their permanent home. If fading or isolation at any point leaves you unsure whether this is stress or the start of illness, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.