It really depends on what kinda water your starting with (hardness) and what to do you want to bring it down to?
Peat does help yes and I used it over yrs for the Killi's. But it can only do so much. So if your source water is superst hard and you want to achieve soft acidic water, you're better up mixing it EACH w/c with R/O water.
But you HAVE TO ensure the mixture is ALWAYS the same. If you expose your fish to constant swings up & down in pH & KH that's a positive killer for them over time.
Test your water exactly what it is (with liquid test kit) and tell us what fish you wanna house in it. You may be far better up leaving it as it is then tempering with it