I have access to RO/DI water. I'm not sure what the PH of the pure RO/DI is, but I remember that I had to use special reef salt in my saltwater tank to get it above 7.8. I think the PH of the RO/DI is lower than that (should be about 7.0). I'm not sure what the PH of my tap water is, since I've never used it for my fish.
I didn't know that about the neons. Would it make a difference if I got them from a small-scale fish store? I'll probably go with the orange tetras anyway, since it seems like everyone and their mother keeps neon tetras. ;)
Any ideas for the one large "show" fish? I'm still not sure what species would work with the bettas, not have to be kept in a school, and be happy in a 29 or 36 gallon tank. I was originally thinking a rainbowfish or a molly, but the rainbowfish are schooling fish and I've read that mollies don't get along with bettas.