Keeping in mind one battery is new but water needs to be refilled and another battery is a year old.
There is a difference between self made distilled water, tap water and battery companies selling their battery Tonic such as AGS. (Can you drink that distilled water of AGS bottle) correct me if u find me wrong.
I'll help later with info.
But point is you are trying to save your battery from impurities. But gravity of battery has nothing to do with it. Still if we remove the problem of impurities. There will be two kinds of waters remaining. One distilled water. Just plane water without any impurity but low gravity. Another is what the companies are selling the battery tonic. Which has 3 parts water and 1 part acid. Leading to good gravity which helps a dying battery cells to keep the level optimum for cells to work.
(I am not talking about fake water or anything).
So if you wanted to refill each cell separately . Those that have 1300 gravity, would be filled with pure distilled water and that cell having 1100 gravity would be filled with battery tonic.
But if this is dying battery then we probably put battery water in all if don't wanna spend too mucch on checking each n every cell. Especially when battery is reducing backup time. But if its new then yea. Distilled water it is.
Thats all the game is about.
Abid saying tap water doesnt mean full of minerals and this n that. Just don't bother much i this regard if UPS is good. Battery will be healthy and a tap water (nalka) will be enough. But if you prefer the exact clear one. Thats good but doesn't make him wrong. When He said to put tap water he didnt mean water from stream.
Hope you get it clear. JazakAllah. Please avoid being sentimental but objective. I'll find more info if you ask.