Im not a big fownd of coral for the next reasons: it creates a too high ph alongside lots of salts and minerals wich many turtles dont like a bit, the botom of the tank can become really hard to clean because of the porous nature of the material and if ingested it isnt as easy to digest as you thing, as turtles chew it can creat sharp edges wich can cause irritacion of the gutts walls and if the turtle is a grownup one it will simply join the peaces together in the gut and form a kind of wet cement insted of digesting (it is a grown up so doesant needs more calcium for its squeleton) and have reall trobble going to the toilet. Also to much calcium is as bad a too low, turtles keeped in such circunstances are prone to kidney problems including kidney stones wich aint a good thing to mess with. I know you have at least one fly and you can use coral as a buffer and its one of the turtles that likes hard and even ligthly salty water but its better to use it in the filter chamber or in another plance in wich the turtle doesant have acess to and use normal river sand as substrate wich also allows the turtle to burrow. Oh and by the way enzimes digest food, aminoacids dont