You can definitely use tap water that is hard with a high pH. I recently had 9 discus tanks set up with four or five pairs going, and I mostly used our tap water which has a pH of well over 8. I had both tank-raised and wild caught fish, juveniles and adults.Two problems with hard water with a high pH:
1. as Pufferxtreme and Dkarc said, hard water and a high pH give you result in a very poor fertilization rate. It's theorized that it has to do with the membrane on the egg being tough for the sperm to penetrate, but it could be weakening the sperm too. If you want babies, you will have to use soft water, which for most is reverse osmosis. (I made all my water changes on my breeding tanks with RO or rainwater, until the babies were freeswimming, and then gradually switched to tap water.)
2. High pH results in more ammonia in the water. Low pH causes the ammonia to stay in a safer form (ammonium ion?), but high pH causes it to be toxic to fish. This is the reason many people use soft water for discus of all ages. Discus take meaty foods which will foul the tank water very quickly. In a tank with low pH it is safer than a tank with high pH.
On a personal note, even though I am arguing that you can use hard water with a high pH for discus, I got rid of my discus because I was mainly interested in breeding and I got tired of dealing with the RO and rainwater. Now I am keeping Central American and African cichlids and never worry about the water, except to add dechlor with the changes.
I attached a pair of fish with a spawn on RO water, but they and their babies spent most of their time in hard tap water. The male is a wild fish and the female is a giant flora.
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