Normal Aquarium conditions would seldom be found outside the green area.
Acidic water loving species such as Discus and South American Cichlids would be happy with a pH of 5.5 to 7.
Alkaline species such as Rift Lake Cichlids would be happy with a pH of 7.5 to 8.5
Most other Aqaurium fish would prefer water that is close to neutral.
dH or degrees of Hardness
0° - 4° = Very Soft
5° - 8° = Soft
9° - 12° = Medium
13° - 19° = Hard
20° & up = Very Hard
Fish that prefer Soft water usually prefer Acidic water.
And Fish that prefer Hard water also prefer Alkaline water.
But most fish will do well around Neutral pH and Medium Hardness.