Using De-ionized Water to Lower pH
I've been looking into keeping Apistogrammas however my local tap water has a high pH and is relatively hard. I understand Apistos prefer softer, acidic water so thought that I could achieve this by mixing my tap water and de-ionized water in a set ratio to reach a desired pH.
I take A level chemistry and calculated that in my 8.5 litre water change I would need 7 litres of tap water (pH 7.8) and 1.5 litres of de-ionized water (pH 6.4). I tried to do this today for my first water change however after testing the pH of the water I was about to put in the pH came out as 7.8 still.
I use test strips which I know can be inaccurate so thought that this may be the reason for the high pH. I have also read somewhere that hard, alkaline water contains buffers which resist changes in pH, is this true? If so, what method would you recommend to lower the pH and soften my water? I don't particularly want to invest in an RO unit, as I currently don't have the time or money to arrange that.
Thank you for any help!