[Skimmerking]

Well some tap water is good and some is down right awful. Its going to depend on where you live. and where you are getting your waster from. Our waster in Brandon ,MB is the crapps my tds reading is at 700PPM or more. Doug's water 5 mins away is like 300+ so there you go.

\Having a RO unit is good it will take out all the silica's and minerals in the water to allow you to have 98% pure water.

With RO/Di water it will allow you to take out all the bad and give you 99% pure water. keep in mind with RO water if you have a ALK problem in the water it will set you back from the high ALk in the water RO units won't take out the ALK.

With RO/DI it will take out the ALk in the water and it is used mostly by alot of the reefers in this hobby.

here is a small article that i found on oz reef

The abbreviation RO stands for Reverse Osmosis and DI is short of De-Ionisation, which are both techniques of purifying, removing the impurities from, tap water so that it is suitable for use in a marine aquarium. RO/DI is using both techniques, first RO then DI, to obtain the water. RO by itself has difficultly removing some of the ions from the water, so typically that treatment is followed by DI to ensure the last lot are removed.
The difference between RO and DI water is the technique used to remove the impurities/ions. RO uses a semi-permeable membrane which is fine enough so that essentially only water molecules will pass through it. As a result you produce about 3 times as much waste water as purified water. DI uses ion exchange resins, which exchange the ions in the water, such as iron, magnesium, copper for hydrogen and nitrate, sulphate, phosphate for hydroxide. DI is fine to use, and if done correctly is better quality than a straight RO system (without a DI on the end).