Not a fan of bottled water but imagine if delicate Neon Tetras are fine; it should be OK for frogs. How much water do you replace every two weeks and what are Nitrate levels before water change? Did Ammonia and Nitrites tested at zero? How old is your test kit and is it within periodicity?

If frog is bloated it could be cause by various reasons; but can't point to any particular one right now. Internal bacteria can do it by attacking organs regulating body water (Kidneys & Lymph Hearts). Bacteria are contagious and only one frog is showing symptoms so it's not a likely cause. Another cause is TOS due to bad water chemistry acting as toxins to frog. Again; that would affect both frogs. Another possibility is an internal disease (i.e. cancer) affecting the previously listed organs.

Only thing can recommend is to move the bloated frog into a separate hospital tank and add 1/2 tsp of Epsom salts per gallon of water. If that hospital tank is unfiltered; change 50% of water volume daily and replace the removed Epson salt according to volume changed. Good luck !