Water pH

[ChipMolly]

Given my variable success with growing orchids at home in the past 2 years on my apartment balcony in Sydney, I thought I might test my water pH. I use a triple filtered system rather than tap water. I bought one of those cheap pH meters which come with calibration powder sachets you make up. Seems to work quite well. To my surprise my filtered water is around pH7.8. Tap is pH8.0.

I have read somewhere including here I think, that nutrient availability (Ca, Mg etc) is optimal at a slightly more acidic pH range, 4-6. Is this true?

If so what's the best way to adjust my irrigation water pH?