Keep in mind the weathered bricks you find have been exposed to rain and sea water for a long time. That should have washed out any water soluble materials. Bricks are generally fired clay. The firing process should also make them safe. So they are probably safe. However they probably have some salt in them. I would collect them and wash them and then put then replace all the water with water that has a known GH, PH, and KH values. Wait a week and then recheck the GH, PH, and KH. If the numbers have not changed they are safe to add to the tank.
When you are at the beach keep an eye open for weathered glass. The resulting glass rock can sometimes be quite colorfull and also might be worth adding to the aquarium. Other than salt from sea water, glass is safe.
