It suggests that your tap water is naturally buffered to 7.0, or you could have some bad pH down (I've seen it). I would try reverse osmosis water. You can probably get it cheaper than distilled at the supermarket. And don't try to keep it at 5.8, just a range of 5.5 to 6.5. Try using a pH down product that was made for hydroponics rather than aquariums, it will stabilize pH better.