[JustOneMore20]

Your pH is fine....its best when its stable, so I wouldn't try to alter it.

How often do you change the water and how much?

Common plecos are more carnivorous I believe, so you should supplement them with some bottom feeder pellets or wafers. They can get over a foot long though, so I don't suggest Commons for your tank. A Bristlenose or Rubbernose pleco, supplemented with algae wafers and bottom feeder wafers should do well.