1 comet needs a 50 gallon tank, so unless you have a 50+ gallon tank, you shouldn't add any more fish. They can grow to 12+ inches long.
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Holly is totally wrong. "Neon glow fish" (which I'm assuming are neon tetra's or glow fish) should not live with goldfish. They are tropical fish and tropical fish need warm water. Goldfish are from Asia, they're coldwater fish.
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Yara is wrong as well, 10 gallons is MUCH, MUCH, MUCH, too small for ANY type of goldfish. 10 gallons is not a lot of space, 10 gallons is tiny.