Shall we put in other terms?
Was it better for Nessie to be crammed in an over crowded tank very likely being stressed and out competed for food by the mollies? Or was it better for her to come home with you to a settled existence with appropriate food and time taken to ensure she got to eat? I know which outcome, although short I believe is better.
I have had quite a few rescue animals (mostly wildlife) and even though they often die while in care, either because the injuries where to great to survive or had to be put down by law because it would never recover well enough to be released back into the wild. I at least know the animal while in my care had safety, shelter, love and positive vibes and even when it dies either on its own or with help from a vet, its soul is sent off to its afterlife still with love and blessings.
All animals are never around long enough for our lifespan, I believe it was to teach us loving unconditionally and losing with grace and compassion.
I am sure Nessie is still smiling at you, and later when you see photos of her smile it will bring back the happy memories of her.