Don't bring you water below 6. You want 6.8. If your tap water has pH 8- yikes, that's not great! Town water should be neutral.

I've never seen or used promix. So I don't know how effective the buffer is or how long it lasts before it's all buffered out. But regular leaf compost buffers in the 'happy cannabis' range. And if you stick with organics, it should stay there for your entire crop.