I'm about to get a 55 gallon tank. I have one 5" goldfish, and I was thinking of adding one or two other big fish in the tank with him (maybe 6 schooling fish too). But I know goldfish are territorial...
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You can do cooler water schooling fish, but they may be eaten by the goldfish.
I'd look for a 75 gallon, not a 55. 55s are not wide enough. They're only around 13 inches wide. 75 gallons are much wider.
You should get more goldfish. In a 55 you could have 2-3 goldfish, in a 75 you could have 3-4.
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You can't add that many fish in a 55 gallon tank. If you are putting a goldfish in, you also need to have double filtration. You need either two 50 gallon filters or a 100 gallon filter. Keep the 5" goldfish. If it is a fancy goldfish, you can add two more to the tank. If it is a common goldfish, you can add one more to the tank. Goldfish grow very large and are very dirty fish that have different water requirements than many other fish you would probably be interested in putting in.
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ONLY GOLDFISH can be with other GOLDFISH in the tank!
NOTHING ELSE!So if you have 1 fancy goldfish you can add 2 more but you must have a GPH of 500-550GPH with your filters combined.