I used my pH meter in some regular tap water, just messing with it, and it read 8.5 pH. I thought I needed to calibrate my meter, but when I put it in the 7.0 pH reference solution it read 7.1. Tap water is weakly alkaline in general I discovered. It does sound like you are using a lot of pH Down though, although you might just have smaller drops than me. It takes me 3 drops of pH down to drop a quart of my tap water to 5.5-5.8 (but I'm doing hydro, not soil; I think you want a slightly higher pH than that for soil). So I guess it would take 12-15 drops per gallon for me.