Why won't my ph stay down!?!?!?

my plain tap water will NOT stay at 5.8 for more than an hour.... 100ppm out of tap 7.0ph i need to ph down A LOT daily... WHY?

just go to the store and buy some distilled water and then test it by dipping the ph dip sticks into the water. should be around 6.0-6.5

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.