Re: Need help on cycle
Since you will be adding fish, the amount of organic acids will be increased. This will pull your pH down somewhat, but it is difficult to tell how far the pH will be pulled down, and the length of time it will take for this to happen. Since your fish do prefer a pH of 6-6.5, I would try to set your tank water to this pH before adding the fish. You may simply need to dissolve some KH, since the KH of your tap water is presumably 4 degrees. You could use a little Acid Buffer in order to dissolve some KH to lower pH to the range you would like.
You can certainly use Discus Buffer by itself to lower the pH to 6.0-6.5. I would start off by using 1/4 the recommended amount, and see where the pH lies. Since you do have KH present in the tap water, you may need to add a small amount of Discus Buffer daily in order to stabilize your pH to your desired range. The phosphate based buffering system will take a little time to overcome the KH that is already present. But once the pH has stabilized, you should only need to add Discus Buffer during water changes.