means there are a lot of things other than water in your water. TDS
is a measure of condutivity which is directly related to what all is in the water besides water since water itself will not conduct electricity. This means more nitrate, silicates, phosphates, etc. All of these are unwanted in a reef tank or if you spent the money on an RO
system. VERY low numbers (single digits) may be as good as some system can get, or at least they don't stay at 0 long. When the TDS
starts to go up it means one or more of the stages of the RO
filter are exhausted and need to be replaced.