Welcome Paulo to the world, some would say addiction, of aquaponics.
Is pea gravel better than hydroton??..... probably only as a factor of cost...
Here in OZ most members battle issues of high temperatures rather than cold... unlike many of the members from either the US or Canada....
For this reason most members in your (extended) neck of the woods are doing their systems either indoors or at least undercover and heated...
But it's very doable.... we even have several Alaskan systems.....
Feel free to post your ideas, drawings and pictures on the forum... you'll get lots of feedback and ideas.
Again welcome.... p.s.... maybe turn your Caps Lock off when posting... makes it difficult to read long posts
