1st time Black Moor owner needs answers!?

Your tank is WAY too small for your fish load. I've had plecos grow 10 inches in a years span. Goldfish are very messy eaters, and will foul up the water in a 10 galloon overnight. Letting tap water sit out overnight, does nothing. There is heavy metals in tap water, as well as chlorine, that wont leave the water, just by sitting out. depending on where you live,some water conditioner still wont get all the bad stuff out, like phosphates. I wouldnt mess with the PH, big fluctuations in PH can be catastrophic, in any sized tank. And with a small body of water such as a 10 gallon, the PH swing will be huge.