Article in English: Hopefully, if you have more than one goldfish, your goldfish are all the same type. Unfortunately, goldfish are known to eat other, smaller fish, and can overeat, keeping food from their peers. If another fish is smaller or slower, it doesn't stand a chance. You can use a commercially available tank divider to keep your “bully” or weaker fish apart from the other(s). Goldfish can be decent "community tank" fish. However, good roommates need to be selected carefully. White Cloud Mountain Minnows or Zebra Danios are good choices, and so are Plecos. However: These fish live in schools, so if you're buying extra fish, you need to at least buy an extra half dozen. So in short: Keep your goldfish with other similar goldfish.  Any new fish brought into an established aquarium should be quarantined for two weeks beforehand. If they have any diseases, you don't want those spread to your healthy fish! Keep in mind that goldfish like colder water than most community fish, so any other species you add to the tank should be hearty. (You could also consider adding a goldfish to a tank with overly-productive livebearers to eat the unwanted offspring and keep your fish numbers in check.) Remember that test you did before you added your precious little fish? You gotta keep that up! Ammonia and nitrite levels should be at 0. A range of pH 6.5-8.25 is fine.

Summary in Vietnamese:
Thả cá vào bể Đo nồng độ a-mô-ni-ắc, ni-trít và pH.