oh to keep pH on a lower level than 7.0 you mith need to purchase another tests (pH) and pH down drops which contain amongst others phosphorous.
I recently invested in 25 6 in one tests. The kH
level is as far as I remember responsibble for stability of pH. So in general is hard to decrease pH of highly mineralised (hard) water. 6.5pH should not make other - alkaline (7.0) fish very unhappy. In such an application an external - fluvial filtration might be easier to set-up
Oh.. 'Twas on tetras, while they according to what I found, originate in Amazon river basin. And I actually am quite familiar with habitat there
Ask then while I now about reproding it and finding a compromise between amazon and tanganyika lake in one tank which is going to disqualify few specimen but would allow me for others ;>