Hi,
You asked:" With fresh water if my PH was low but my fish were healthy and active, I didn't worry about it. Is this the same as with saltwater."
As I understand it, in the simplest terms:
With freshwater fish, As long as you don't use straight tap water (chlorine) you can get away with almost anything. Lakes and streams are shallow and change all the time. The critters can stand a great deal before it kills them. Over the centuries they have learned to adapt to change pretty fast. (not amomonia or contaminates like soap)
The ocean is very deep, lots of waves and it has remained stable for a very long time. The critters from the ocean are not used to change. They have not adapted in that area.
This is why a tank raised fish or coral is stronger. They have grown up with slight changes because a fishtank is not an ocean, slight changes happen and they have gotten used to it, to some tiny degree.
So your PH, Alk, nitrates, nitrites and calcium are all very important to get stablized. Nature has not prepared the ocean critters for change.