Ph is really tricky. Fristly what is the Ph from your tap ? Also do you know your Kh level ?
Some fish prefer lower PH, some like it around 8. As your Ph increases so does the toxicity of ammonia so thats one problem. The other is that depending on your buffer, its hard to changer your Ph. Normally Ph does decrease over time. Normally livebearers like water between 6.5 and 7.5 but as long as your weekly maintenance is good, they will survice like that.
How often do you peform water changes ?