Young Goldfish die with water change

[seemor]

Hi, I have been keeping goldfish for over 5 years. My 40 gallon tank has three, their size is about 4-5". They are healthy and very active.
My granddaughter and I started a 20 gallon tank and put in two popeyed black mollies. They did fine until I did the normal water change. I have done nothing different than I have in the pass five years. But one by one they died. My granddaughter is so sad and before I get more any help is welcome. They were about 1-2" and that is the size of the ones that are now five years old when we first started out.

Seemor

[Oscar_freak12321]

Sounds like your tank wasn't cycled. Goldfish wouldn't die randomly without reason. What are your water params? Temperature?

[robflanker]

did you forget to add dechlor this time?

[Wild Turkey]

Did you choose a cycling method? (the most wise one for your situation would be seeding the new tank with some media from the established tank's filter, the same day you add fish)