Your installation shows careful planning with steel angle supports and rubber insulation to prevent abrasion. However, the tank dipping and inverting when filled suggests a few potential issues. Consider reducing the spacing between your support brackets from 16 inches to provide more uniform support and reduce flexing. Ensure the tank is level and the floor joists are even to promote even weight distribution. While rubber sheeting helps prevent abrasion, using firmer or additional support material can limit tank flex under load. Lastly, review the tank manufacturer's installation guidelines for recommended support structures. If possible, sharing photos can help provide more precise advice.