Only distilled or purified water should be used to fill batteries. Tap water, drinking water, softened water, or well water should not be used because of possible impurity and mineral contents that can negatively affect the performance and life of the battery.
As for the correct water level, each brand and size battery is a little different. If you have one, refer to the owner's manual. If not, most of the ones I have had to fill it was "full" once the water reached to bottom of each fill tube.
**Always fill it AFTER you charge it otherwise you risk boil over.
More importantly, Thank you for serving.
A-Jay