Goldfish and harlequin rasboras start with opposite needs
Goldfish and harlequin rasboras should not be combined, because almost every requirement points the other way. A harlequin rasbora is a small tropical schooler from warm, soft, plant-lined streams, happiest in a group drifting through heated water. A goldfish is a large cold-water fish from an entirely different tradition of keeping, needing unheated water, generous volume, and filtration built for heavy waste. In one tank, the rasboras would live chilled and stressed at goldfish temperatures, or the goldfish would live overheated at rasbora temperatures, and neither outcome counts as care. Add the feeding difference, where bold goldfish take food at every level while a rasbora school feeds delicately in mid-water, and the smaller fish lose on temperature, nutrition, and safety simultaneously.
A harlequin school is within a goldfish's reach
Predation risk for harlequin rasboras under goldfish is real, even though harlequins are deeper-bodied than many small schoolers. That deeper body protects them somewhat from a medium angelfish, but a mature goldfish is a much larger fish with a much wider mouth, and a rasbora that is young, sleeping, or weakened sits within its capacity to swallow. Goldfish also investigate tankmates persistently with their mouths, so rasboras face chasing and mouthing long before any fish is actually eaten. A school under that pressure stops displaying the relaxed, spread-out schooling that makes harlequins appealing, and instead bunches tightly in cover, feeds poorly, and fades. Losing one fish at a time overnight, with no sign of disease, is the pattern owners of this mixture typically report once the goldfish reaches adult size.
Why plants and hiding places cannot fix this pairing
Cover helps shy fish against many tankmates, but plants cannot repair what is wrong between goldfish and harlequin rasboras. The temperature conflict operates everywhere in the tank at once, so a rasbora hiding in foliage is still a tropical fish sitting in cold water with a suppressed immune system. Goldfish also work against the planting itself, nosing through substrate, nibbling soft leaves, and uprooting the stems the rasboras would shelter in, so the cover steadily thins over time. When every element of the setup, from temperature to planting to flow, has to fight the pairing, the honest answer is that the pairing itself is the problem to remove.
Building the two communities these fish deserve
Goldfish and harlequin rasboras each shine in a community designed for them, which is the constructive way to view the separation. Harlequins in a warm, planted tank with other small, peaceful tropical schoolers show their copper colour and wedge markings at full strength and school openly through the middle water. Goldfish in a spacious unheated tank with similarly built goldfish companions can be fed properly, given room for their eventual size, and kept in water that stays oxygen-rich and clean between changes. If the two are already mixed, move the rasbora school intact into the heated tank, since splitting a school adds stress to fish already coping with cold, and then watch for delayed losses over the following week as chilled fish sometimes decline after the move rather than before it.
One household record for two very different aquariums
This mismatch is easy to create accidentally when a family runs several tanks and a child or relative adds the pretty copper fish to the big goldfish aquarium as a surprise. Recording each tank's temperature type and full stocking in a single shared place, with a plain note that tropical schoolers never join the goldfish, stops that surprise from happening. KinsLe Family Sharing gives every member of the household that goldfish tank record to check before any new fish comes home.