There are many others that have an adult GT in a 55 without issue.
Doesn't mean that the fish are happy or healthy. Green Terrors are pretty big fish and can truly be Terrors in my experience. I would say 75gallon minimum for 1 and a couple small friends. 55gallon might seem fine now since they are small but they will get bigger and you will begin to see that 1 is cramped let alone with other decent sized fish.
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