Planted Tank Enthusiast
Join Date: Aug 2006
Location: Cincinnati, Ohio
As imeridian said, RO water should have a 0 GH/KH. At the very least, it shouldn't be testing a GH of 4 and a KH of 5. That's way too high for RO water. Bring it to their attention. With the volume of RO water that an LFS might sell, I can understand how it might be easy to forget to change filter cartridges/membranes/resin or whatever.
How are you testing your water hardness? In my experience, the API GH/KH test kit make me want to throw the things out the window. I'm sure they're atleast semi-accurate, but I can't tell for the life of me when they change colors. All I've been able to determine is my water is 10-degrees-of-I-want-to-run-this-test-kit-over-with-the-car. (lame joke, sorry).
What I use now is the API test strips. They're more expensive, less degree of accuracy, but they give me a nice color indication of GH/KH, so I get a ballpark idea.
If you're doing RO for more than a small aquarium, I'd seriously consider getting your own RODI filter and some sort of storage bucket (I've heard the Brute trashcans work well). I paid $140 for my 6-stage RODI unit, which is really nice. $140 would be 280 gallons of RO for you, but this unit will make well over 600 gallons of RODI before anything needs to be replaced (even then, it's just the resin I believe).
Why do you need RO water? What's your water like out of the tap? Mine is compareable to liquid rock, but if your water really isn't that bad, you might be able to do without it. If tap water isn't so good, a nice RODI setup would do you well.
Just my $.02.