RO/DI refer to water that has gone through a Reverse Osmosis/ De-Ionization process, which your LFS might have done...call them up and ask them if its RO or RO/DI. You can buy an RO/DI unit from places like
www.thefilterguys.com
A TDS test will test for water pollutants or basically how pure the water is for your tank. You want a zero TDS reading.
What type of filtration do you use? Protein skimmer? Bio balls? If it is Bio balls than there is your problem.