This kindof a hard one, yet an easy one at the same time.. TDS or "Total Dissolved Solids" are things we often times can't test for, and, potentially can cause problems in our Reef tanks. A few examples of the S in TDS are phosphates, silicates, pestacides etc. Phosphates are often associated with nusance algae blooms. Silicates can also contribute to algae and dinoflagellate blooms. Pestacides, well, kill stuff.
By starting with 0 TDS, we are greatly increasing our chances of success. I.e. only adding things to our tanks we can control or test for. In Anaheim where I live, we use city well water. Various wells have different TDS ratings. Specifically, by my house the water comes out of the tap at a rediculous 1200+ TDS. Yes, you read that right... The city averages different Wells TDS number to meet the Federal mandates of under 900.
There a whole slew of other things making up that TDS number. Some not so bad for us, some really terrible for our tanks. I have been making water at home for the last couple of years, and my tank has been better for it. Trusting water you get from a store just because "It came from a fish store, must be good" is playing with fire. I know full well that there are many stores out there that truely do make 0 TDS water, but many don't. Tap water, well water, or any other source that you can't confirm the quality of is, in my opinion, a horrible idea.
We have all spent hundreds if not thousands of dollars to keep a beautiful piece of the ocean in our homes, why risk all of it on risky water. There are plenty of relatively inexpensive RODI units available at places like BRS or Marine Depot. Being able to control every aspect of what goes into our tanks goes a long way toward long term success..
That said, make sure to know whether your city uses Chloromine versus Chlorine to make sure to get the correct carbon block. But that is another lengthy diatribe...
Or you can look up the BRS video about RODI units and the Chlorine/Chloramine difference...
