Okay... I think I know what your issue is. Your tank is cycling and the pH is decreasing because of the nitrogen cycle. I could get into the whole reaction but basically as ammonia converts to nitrite, pH goes down.
When you cycle an aquarium, you initially get an ammonia spike (which you did). Eventually, bacteria flourish to convert ammonia to nitrite, so your ammonia goes down, and nitrite goes up (check). Eventually bacteria will convert that nitrite to nitrate. In the meantime, pH steadily decreases. This leads me to...
Two important points:
1. You need to do water changes, not just top-offs after evaporation. When water evaporates, what's left in your tank becomes more concentrated, so your pH goes down even further, and your nitrates accumulate. Routine water changes are a must. They keep your nitrates down, and also they keep your tank water's pH from dropping too far below the tap water's pH.
2. As said before, it's much more important to keep your pH consistent rather than aiming for a specific value. Your JDs will be just fine anywhere from a pH of 6 to 8, but if the pH changes quickly it can be harmful or even fatal to them. So just use your tap water with no chemicals (except a good dechlorinator of course), and do weekly water changes to keep everything routine.
Does that help?
