I have not changed my filters in my 4 stage RO system, but was reading nearly 0 ppt for phosphate. Today we changed the filters and also went to 6 stage. One pre-filter, two carbon filters, and two color changing blocks. Now when testing with TDS meter, the one on the machine and a handheld unit, both are reading 16 ppt. What is wrong? Did I do something, do i have the incorrect water flow between each chamber. Your input will be helpful.