Colloidal Silver Health Benefits

We have been making CS by the gallon, and just recently watched the Earth Clinic videos on how to make it. We do everything as directed, check the ppm with a TDS meter. We use the electrolyte solution and the reducing agent to get the CS. We get the correct color of the CS. The CS will drop in ppms dramatically as it sits overnight, say from 18ppm down to 5ppm, though. Has it lost potency, or is there a different gauge or meter that we need to test the potency of our CS?