I kept a florida softshell with fish for 5 years - it grew from 2" to 13" carapace length - I housed it in a very large tank and everyso often it would bite a fin or a small piece of flesh from the fish - eventually it turned into a fish killer and then had to go .
Yours could well be an african softshell - very nice if it is too - beautiful - I wouldn't worry about the hi pH, it will do it the worlkd of good - as long as it's not too extreme.
Try and get it eating pelleted food as soon as you can - I have found that turtles that eat pellets are less likely to attack fish - initially
carl