Is there an action replay code for 999 heart scales in Pokemon diamond?

yes, its 94000130 fcff0000 b21c4d28 00000000 b0000004 00000000 00000890 03e7005d d2000000 00000000 just press l+r it will take away the first item in you items pocket though, so make sure you dont care to loose it hope it helped!!