Nitrates will not make the water acid, as it is caused by hydrogen ions and they are neutralized by hydroxyl ions from things like caustic soda or by reaction with things like carbonate or bicarbonate which act as buffers. You may be producing acid waste along with your nitrates however. pH is logarithmic so if it takes one drop of acid to turn water from pH 7 to pH 6 it will take 10 drops to go from 6 to 5 and 100 from 5 to 4. pH round 6.5 to 7.5 are very small differences and to try and maintain them is what keeps the shops rich and you poor. Fish are generally happy with constant pH and realy don't ike constant changes as people chase a particular and only theoretical pH. It is useful to have particular conditions when breeding but not so important otherwise. I keep killies in tap water and breed them in rainwater.