We discourage tires and forbid metal (boating hazard on low water). As a matter of fact, we removed hundreds of tires on Overcup and Harris Brake during the last drawdown. This was once encouraged by biologists years ago but we found out (years ago) that tires can leach bad chemicals into the water. Natural materials, plastics, pcv, are fine as along as they are not placed in areas that cause boating hazards. Natural materials seem to attract more fish but artificial last longer. As long as you aren't just sinking junk and calling habitat, you are fine. Also make sure that whatever you are sinking is securely anchored. The above rules apply to AGFC lakes. Consult your local Corps office for their dos and donts.