The pH of carbonic acid in normal atmospheric conditions is about 5.5. The pH of your tanks suggests that either pH test is off or you have stronger acids at work in your tank.
I have ADA aquasoil. O kH and 0 gH.
Its right out of the tap. However, it must also be noted my water is slightly cooler than the average. Around 68-65 which means it can hold slightly more dissolved gasses than the normal room temeperture. So with that combined, I think it draws down my pH a little more.