Originally Posted by tadpole
The submersible pumps designed for Water Feature applications are either stainless steel or titanium (for salt water apps). Bearings may be of a different metal but they are not exposed to water, so it doesn't matter. And, yes, the reason behind this is corrosion. In any Water Feature, the goal is to have the clearest water possible, free from suspended matter and/or discoloration. Many submersible pumps today have an ABS housing. Water pH is the main determinate as to the rate and amount of corrosion.
I would also like to clarify another point. Pump vaults are not constructed to bear the weight of any Water Feature (fountain, bubbler, etc.). They are designed for the sole purpose of protecting the pump from large debris, (read gravel or rock), while allowing sufficient water to reach the intake of the pump from the surrounding reservoir. A pump vault's access for maintenance is through the top.
So you would never use a metal grate in a water feature with no "aqautic life."