Dealing with pitting/oxidation on our submerged Aluminum fixture for lumbar implant device.

I am a technician at a medical device manufacturer. We are using a fixture made from aluminum to fatigue test our product idea, a lower lumbar implant device. The fixture is submerged in water so we can maintain a constant body temperature, 37.4C. I've tried using Machining coolant but the material continues to form small white "pimples" on the surfaces which are submerged, as well as gradually turning dark gray. Is there a compound I can add to the water bath to prevent, or reduce this occurence?

Thanks for your consideration.
Jay Markwart

As you've found out, aluminum is not a very corrosion resistant material. If it's not too much trouble, why not remove the fixture, clean it, and paint it (or some other coating)?