The water here has a pH of around 7.5... How can I lower it to 7 (or even 6.5+)safely without screwing with the plants? is vinegar fine?
Vinegar, lemon juice, and anything else that is acidic will lower your Ph. It may not be as consistent with it's results as Ph Down would be, but it's safe. By consistent I mean it'll take some experimenting to see how much you need to get your Ph down.
I'd recommend calmag +. It's not exactly a PH-down, but if you're not already using a calcium-magnesium supplement and have soft water, it's probably a good idea and may be enough to get your ph down to where you want it.