it out of the water, drained and cleaned the reservoir, and thought about what i could do to keep the air-stone down, without introducing rust into the system which would potentially disrupt the balance of the nutrient solution.
the answer, as it often is, was zip ties! i love zip ties. i put a zip tie through the suction cup and around the hose as it went into the back of the air stone. i added another zip tie around the air supply line just behind the other to prevent the first from sliding down the barb of the air-stone and falling off. 