Temperature and appetite rule most choices out
A goldfish eliminates most tank mates before temperament is even discussed. It is a coldwater fish that thrives in cool, oxygen-rich water where tropical species shiver and sicken. It is also a large, enthusiastic, permanently hungry omnivore that produces waste like a fish three times its size, and it investigates the world mouth-first: snails, shrimp, small fish, and soft plants are all sampled. Any companion must therefore pass three tests — it must want the same cool water, tolerate heavy filtration and mess, and be too big or too quick to be eaten. Most of the tropical catalogue fails the first test alone, which is why goldfish tanks look different from community tanks.
Other goldfish: matched by type, not just species
The companion a goldfish is built for is another goldfish — but the variety matters as much as the species. Commons, comets, and shubunkins are strong, fast swimmers with single tails; fancies — fantails, orandas, ranchu, telescope-eyes — are slow, deep-bodied, and in some cases limited by their own breeding. Mixed together, the fast fish reach every pellet first and the fancies slowly thin in a tank that looks well-fed. Keep like with like: single-tails with single-tails in the biggest, most filtered water you can give them, fancies with fancies of similar swimming ability. Two fancies of different varieties make a charming and fair pairing; a comet in a fancy tank is a slow-motion problem.
Coldwater companions some keepers succeed with
A short list of non-goldfish companions has a genuine track record in large, unheated aquariums. Weather loaches — dojo loaches — share the goldfish's love of cool water, dig happily in sand, and forecast storms with restless swimming; they need a group, a lid, and room. White cloud mountain minnows suit unheated tanks too and are quick and hardy, though in a small tank a large goldfish may eventually snack on them, so the pairing belongs in spacious setups. Rosy barbs tolerate cool water and hold their own. Each of these works only at goldfish scale — big tank, serious filtration, cool temperatures held steady — and none should be added to fix an algae problem in a tank that is already full.
Tank mates that cause real harm
Some pairings fail actively rather than passively. Common plecos are the notorious one: sold as cleaners, they grow huge, want warm water, and have a well-known habit of rasping at a sleeping goldfish's slime coat, leaving wounds on a fish too slow to escape. Tropical algae eaters generally want warmth the goldfish cannot offer. Tiny fish — danios, guppies, small minnows in a small tank — become expensive live food. Aggressive or nippy fish worry fancies whose fins trail like banners. And invertebrates are snacks with shells: shrimp vanish, and even large snails get harassed by curious mouths. If a pairing requires you to hope the goldfish behaves unusually, it is not a pairing.
Space: the compatibility factor that outranks temperament
More goldfish combinations fail on gallons than on aggression. Goldfish grow — commons well past the size of a hand — and two fish that tolerated each other in a modest tank become stressed, messy competitors in the same water a year later; water quality, not fighting, is what usually breaks the peace. Plan companions for the fish's adult size and adult waste, in the largest and most filtered tank you can maintain, and treat every addition as permanent: rehoming a foot-long comet is nobody's easy afternoon. A goldfish's long life is exactly why its paperwork matters — its KinsLe Pet Passport keeps its companions, tank history, and health notes in one packet that can span the decades this fish may well be with you.