hardness

Which is softer when broken into smaller pieces and put in water?

I usually do two 7.5 percent water changes a day, once every twelve hours or so. I change out about 30 gallons of water a day. By doing this, I can keep my nitrates at 5 - 15ppm, but the hardness is very low, at 0 - 0.25ppm. Any suggestions on how to keep it a little higher without cutting down...