After the Team Meteor Raid of Charlottes little hideout in Tanzan Cove you will need to head to Team Meteors Base in Tanzan Mountain to continue the story after you reach it go to this area and talk to Simon.
If you didn't talk to Simon there you can still talk to him in the Glass Factory, he will be inside this room. Edit
After that continue the story, after you're done at Tanzan Mountain go back to the Beryl Cemetery Simon should be there, talk to him.
After he tells you his story go to this apartment in the Peridot Ward the building is right of the Pokemart.
After entering the building Simon will be sitting in the sofa to the right, talk to him, after that go back to Charlottes Hideout and use Dive to dive under the lake Taras Body will to the left.
After searching the body and seeing that the ID Tag is missing dive back to the surface and go to the right part of the Lake.
By talking to this guy he'll give you a Super Rod and tell you that he can search the Wiscash you fish up for you.
You're probably going to have to fish up at least 3 Wiscash before you find the one that ate the ID tag.
After you get the ID Tag go back and talk to Simon after you give him the ID Tag he will reward you with TM06 Toxic