I thought I was the only one who didn't know this as a newbie....
I had very hard water for my
FW tank...and I was told..."Just get a
RO/
DI Filter". To me...and any newbie..since you have bought you
HOB Filter, or Canister Filter (and probably a Diatomic filter by now to fight the algae brough on by your tap water)...you would assume that this "filter" would also be for the aquarium.
...But it is NOT for the tank...think of it as a filter for your TAP water. Then the nice clean "TAP" water...now
RO/
DI Water... gets added to the tank...and in my case with a bucket.
Most people use
RO/
DI water for Saltwater Aquariums...they use the
RO/
DI ONLY and add their salt. A few people use it who have
FW tanks to "adjust their Tap water". In my case...by pH was too high at 7.8...so I used 1/2 Tap and 1/2
RO/
DI to get the pH to 7.5 or so. Now I use all Tap water for my
FW as the water has great
KH and I used a pressurized
CO2 system to keep my pH at 6.8...and all Alage is kept at bay.
WC