There's a pond by Vaughan that's full of them and some Kois too. I used to catch them on carp rig. Mill Pond I think was the name by Bathurst close to Major Mckenzie.
I used to fish Mill Pond as well. I've read somewhere on this forum that Mill Pond in Richmond Hill is closed to fishing at least by the city
Whereabouts you catching these in the GTA?
The pic looks like Evergreen Brick Works