Depends on the fish. If you keep a fish outside it's natural parameters you gamble some thrive, some die natural selection. That's why captive breed fish are so much hardier, the natural selection already occurred. So you can have discus quite happy in hard 7.8 pH water, though far from optimal and not for the faint hearted.
I use RO water with adatives precisely for it's stability. I have a set tub with a 45 gallons capacity, I heat the water to tank temp, add in seachem GH and KH powder 1 tbs of each. Do this 2-3 times per week, organics are zero, kH around 2 GH around 3 pH is 6.8-7.0. Water changes take about 15 minutes. Don't have to worry about off gassing tap or the changes due to water treatment. The RO is running all day when I'm at work and my water bill is around $150 per quarter.
I use RO water with adatives precisely for it's stability. I have a set tub with a 45 gallons capacity, I heat the water to tank temp, add in seachem GH and KH powder 1 tbs of each. Do this 2-3 times per week, organics are zero, kH around 2 GH around 3 pH is 6.8-7.0. Water changes take about 15 minutes. Don't have to worry about off gassing tap or the changes due to water treatment. The RO is running all day when I'm at work and my water bill is around $150 per quarter.
