Could too much baking soda leave rust stains in a in-ground fiberglass saltwater pool?

No, baking soda will not leave rust stains in a fiberglass pool. Baking soda (sodium bicarbonate) is a mild alkaline substance commonly used to increase the pH level of pool water. It is non-corrosive and does not react with metals like iron to form rust. Rust stains in a pool are typically caused by the presence of iron or manganese ions in the water reacting with dissolved oxygen and forming iron oxide (rust). Proper pool maintenance, including regular pH adjustments and controlling metal levels, can help prevent rust stains.