That is an easy fix. Your LFS
should carry PHs, just have a look and see if you can find one that will fit in your tank. I don't think you are going to want too much flow but some will help you out. I would suggest a submerged PH that goes right down on the bottom where you have the problem. Not knowing how big your tank is but assuming it is FW
I would expect that a maxi-jet 600 would do the trick.