other then your arrows way to small to be in that size of a tank.. even at 4" unless you're doing 25% 2x a week water changes. and are cycling a bigger tank as we speak... he/she will need it. consider taking the arrowana back to your LFS, or investing into a 180gallon + tank in the next year. Your tank stock was way over.. which likely caused your water to be off to begin with. PH is usually my least concern when tanks go bad.. as long as it's steady a vast majority of fish are fine in either higher or lower PH then what they "should be in" .. Nitrites and Ammonia are the biggest killers.. and Nitrates over 20ppm can cause issues.
But I agree your knife likely got an infection due to ammonia burn/nitrates, scaleless fish are very suceptible to infections.
As sensitive as Arrows can be to changes.. I would leave it in it's current tank, are their any other fish left? Maintain 25 - 30% WCs 2x a week, make them as stress free to the fish as possible. Keep the light off. and let things settle down.
I would treat for bacterial and if you don't see a noticable improvement in a few days, start w/ fungal. avoid overtreating the tank in panic. You can kill a fish by overtreating a tank just as easily as not treating.
