My main water source is tap water, which around here typically has a pH of 7. You might want to consider purchasing bottled water or installing a RO system. Personally I would not use RO/DI in a freshwater system.
How come you would not use RO water in a freshwater system? Is it because of the fauna you keep? As far as I know, it's popular choice for a lot of planted tank keepers. I have just started converting to RO water for my tanks, slowly. I do dose to replace lost minerals, but the tap in my area is very hard and all that calcium and magnesium stains my tanks and my plants do not appreciate it either, especially when I used it in my humidifiier, it would stain my leaves.
So what caused that initial drop in pH for you? Oohhh i just read... the ion brick?
Anyways, it's lookin great, can't wait to see it grow in, and hope you're able to fit some more orchids in there