Why hard water affects your diapers.
OK, to figure out how hard water affects your dipes, you need to understand how detergent works. It basically attaches its self to the gunk in your diapers and then helps pull the yuckies out. This is how all soap works. Which is why it is so important to rinse fully and get out all the water you possibly can from your dipes before drying them. Hard water has lots and lots of minerals in it. Those minerals attach to the detergent just like the yuckies in your dipes, but they make it very difficult to rinse out. They basically steal your detergent from the wash and then get stuck in the fibers. So you have 2 problems here, your detergent not cleaning properly AND not rinsing all the way out. Follow so far? Kay, now how to fix the problem.

The boiling method.
This is what your Gramma used to do with her diapers and is not to be done if there are snaps, velcro, PUL, wool or fleece on your diapers. Basically just prefolds or contours. Get a big pot and boil your diapers. (No detergent or any additives, just water) It will help "melt" the detergent out of your diapers. It will also kill any bacteria you have in your dipes. You will know it's working if you get a layer of gunk on the top of the water.

The RLR method.
These are little balls of diaper miracle cure (just kidding!) It is available of those stips in the detergent isle of WalMart (I have never seen it anywhere else) They are about 1-2 bucks for a pack of 2. Buy at least 3 packs. You are going to wash your diapers once with detergent and a ball of this and then at LEAST 5 more times with no detergent and a pack of this. This hslps the minerals to dislodge from your diaper fabric. Try to do this at someones elses house. somewhere that doesn't have hard water. I do it at my parents or the laundromat. You will probably need to do this about once a month. Whenever you notice your dipes getting wiff. Also, add one of those little balls about once ever 1-2 weeks.

The Calgon method.
Some as above, seems to work a bit better for me personally.

The bleach method.
This is going to be the least well recieved, but works the best. Wash your dipes once with 1/4 of a cup of cheap bleach. About half that for the clorox stuff. Your dipes will smell like bleach when the wash is done, if it doesn't, you didn't use enough. Do another wash without adding anything to make sure you rinse it all out. Then use your diapers. You will be amazed at the fact that the pee DOES NOT SMELL. After that, as maintanance, add about one tablespoon to a wash once a week or every other week, whatever works best for yur dipes. This does not work with oxygen bleach. Just chlorine

The bac-out method.
THE most expensive way to keep your dipes stink free, but works and doesn't hurt the dipes or the environment, after using one of the above methods to strip your dipes, squirt bac out on each one on the soiled area before tossing it in the pail. Works great for poo stains too. BioKleens oxygen bleach works well on poo stains too applied directly to the stain and left to sit a bit, but doesn't rinse out well with hard water. The bac-out (same company) does.

Solutions for soft water
...Not enough minerals sometimes don't "activate" the detergent enough. In that case, don't add more detergent, add a bit of borax or baking soda.