PH = Power of the concentration of the Hydrogen ion
pure water has a value of 7 it is Neutral PH
Battery Acid has a PH of 0 thus lower than 7 PH water is Acidic water
the opposite end of the spectrum is 14, Toilet Bowl cleaners. Ocean water has a PH of 8 PH above 7 is called alkaline.
Fish for the most part aren't very picky about PH unless your trying to breed. That doesn't mean there aren't fish out there that are not sensitive to PH.
Nitrite and Nitrate are what you get when when Bacteria break down Ammonia. So Ammonia (NH3) gets broken down by Nitrosomonas bacteria to Nitrite (N02-) and nitrite get broken down by Nitrobacter bacteria into Nitrate (NO3-). Ammonia and Nitrite are poisonous to fish. But Nitrate in high levels are also bad for fish.
LOL woops sorry SB you type faster than I do!