Steel scuba tanks never float; they are always buoyant. Aluminum scuba tanks sink when filled but float when empty owing to the weight of the compressed air being lost. It all comes down to buoyancy when it comes to how and why diving tanks float. Steel is heavier than water so it sinks. Aluminium is lighter than water so it floats. The reason diving tanks don't usually tip over is because there's nothing in them to tip over. If you put something like a milk crate in a scuba tank and tried to tip it over, it would work exactly like a boat: The weight of the crate would force the tank up into the air, making it more buoyant and causing it to stay upright. Even if you removed all the air from a steel scuba tank, it would still be heavy enough for the bag to keep its shape. But an aluminium tank would collapse under its own weight even before you started filling it with water. Aluminum has many advantages as a material for scuba tanks because it's light, strong, and doesn't react with breathing gas. But because aluminum is so light, it tends to melt if it gets too hot, which can happen if you have a leak in your tank. This could lead to bits of metal getting into your tank's breathing gas, which could make you feel dizzy or cause other problems if you dive from that tank.