Do not use PhDown or any other chemicals to alter the water. The stuff causes drastic shifts that might kill your fish, and are temporary at that. Instead you might want to use R/O or some other purified type of water to lower pH or you can use shells to raise it safely.
There is advice on how to help it but they just try and make you buy more stuff.
Water changes/filtration/stocking/overcrowding/feeding are what you need to fix if there's a problem. Adding chemicals to the tank may make it seem better but it will not fix the problem, just the result.