Unless you wish to do very frequent(3-4 per week) water changes, you need a Kh of 2-3 degrees minimum for a stable tank. If your Kh is 2-3 degrees at normal atmospheric co2 levels, your Ph will be 7.6-7.8. So anything below 7.6-7.8 Ph should worry the heck out of an aquarium owner. Wild water and closed sytems operate under different rules. without constant automatic water changing, you cannot duplicate natural acidic water, and still maintain a stable environment. go for stable match your tank to tap and your fish will be quite happy. The Ph down game is not worht playing.
dave
dave