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If you've kept the existing fish in water with that pH for two years I would not change it, rather change the fish you want to put in to suit.
If you tamper with the pH just to benefit one new fish you will risk losing all your existing ones.
Plus a quick Google search lists the BNP's pH range from 6 - 8 so doubtful if that was the cause of death.
Well Gold Royals can get 12"+ similar to a Common.