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.
The full Q&A library answers the most-searched aquarium questions. Browse calculators, glossary, and disease database for related help.
Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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