Whether fish need companions depends entirely on species + social structure. Obligate schoolers (need 6+): all tetras, all rasboras, all corydoras, all danios, harlequins, white clouds, kuhli loaches (3-5 minimum), giant danios. Without a school they hide, stress, fade, and die early. Loose schoolers (need 4-6): most barbs, gouramis (some), Bolivian rams (pairs), keyhole cichlids. Solitary or pair: male bettas (must be alone, no female except for breeding), most loricariid plecos (territorial), kribensis cichlid (pair), apistogramma (pair). Aggressive solo: Oscar (1 fish 75g+, 2+ in 125g+), red devil cichlid, Jaguar cichlid - very large minimums. Saltwater: clownfish (pair only - never trio in tanks under 200g), tangs (1 yellow tang per tank below 180g, multiples possible 220g+), wrasses (varies wildly). Reef inverts: cleaner shrimp pair up, peppermint shrimp like 3-5, Banggai cardinals school in 5+ in large enough tanks. Rule: research the species' natural social structure before stocking - "1 of each kind" is the worst beginner advice.
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