I'm sorry for your loss. The problem is you take all your gravel out AND 50% of the water... Should be one at a time, with at least 3 days in between for the system to adjust... I'm not sure on Indo or NTT, but widebar is very hardy fish. I had one jump out of the tank for god know how long (there was a 1 hour gap between when I saw him in the tank and when I saw him on the floor of my garage with temp around 50), but he made it after all