Dragon gobies will need about a 55 gallon minimum with a salinity of 1.008.
They have the possibilty of getting around 2 feet, more likely a foot. if your 'planning" on buying them a bigger tank, then that would be a realistic option.
Though, with a pacu, you won't be able to buy a big enough tank for it. A fish the size of you, that weighs probably 90 pounds, eats enough food to feed your family, will never be able to be kept by household aquarists. There's maybe one person I've seen with a school of them in a nice tank, and it was in the 50,000 gallon range. A "local" zoo also has a school of them at 5 feet, along with the larger catfish, in probably a tank that's the size of a small mansion. I once had one "back in the day" in a 55 gallon. It reached about a foot long before it bludgened itself to death against the tank wall, when I was trying to move it.
Sorry to go on a speech there
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