Don't use pine. The resins in the sap have antiseptic properties which we don't want in our fish tanks. I imagine it could eventually be aged out of the wood, but it is so much easier just to find better wood.
If it smells strong, then whatever is in it has not weathered enough to be safe in the tank. Toss it out back and let it sit in the sun and rain for several months and check it occasionally to see how it is doing.
You might hurry the process by boiling it. Alternately boil it and bake it to simulate drying and soaking that happens when wood is exposed to the elements.