Very sad to hear about your battery loss. If one battery is faulty, take it to battery wala, ask him to replace with another working battery of same age, and use it with existing battery.
It is not good to use new battery with 1 year old battery, as it will be damaged soon because of old battery,
Complains in AGS batteries is sky high now a days, the best tested brand is UGS series of PHOENIX.... which is available in UGS, 215, UGS 175, UGS 140, UGS 125.
For your UPS, UGS of 175 is an excellent choice than narada or any other MF battery.
And Best setting for your UPS with UGS 175 is ......Batt Full cut of: 28.40....Battery Low 21V........Charging Amperes 15 Max.....
Never use tap water to level electrolyte, use distilled water only.....AGS battery Tonic is best