As you progress through the arcade mode, the levels get harder and harder, asking you to collect more crystals and throwing a lot more echoes onto the screen. (playstationlifestyle.net)
They would take up room needed for more crystals. (geekwrapped.com)
This food has everything I'm looking for in his diet and it also includes an s / o index to protect him from developing more crystals. (royalcanin.com)