The meter you are considering has a tolerance of 2% of 0- 1999 ppm TDS. This means it could be off by 39.98 ppm and would be useless as you are measuring 1-20 ppm.
You also need a meter that measures in conductivity Microsiemens uS for silver. The Microsiemen reading is very close to PPM for silver. With a formula you can get it closer.
The second meter I showed you from the Silver Puppy website will measure in both us and TDS and has a huge range. It also has three calibration curves so it is accurate at 0- 20 ppm. You could use it to measure distilled water, Colloidal Silver and tap water.