I have 2 goldfish in a tank, my mother wants to move them from the indoor tank to the pond in the garden, she doesn't believe me when I say the temperature change - and change in water quality would probably kill them, or stress them enough to make them highly susceptible to disease, eith of which probably result in their death.

who is right? and what to do?

both the fish are over 3 years old, I don't know breed, and it's somewhat difficult to measure size.

(She wants to move them so she can put some tetras in the tank.)