Some fish are more sensitive than others. That's why certain fish can survive the same conditions while others die rather quickly.
The tetra you mentioned are one of the weaker fish. Guppy, goldfish are hardy fish. And Goldfish is not tropical, so they are the least sensitive to water temperature change. Also the stock might make a difference too. I used to buy from local fish stores and one of the cheaper online stores. They did not last long and often carry parasites and diseases. When I purchased a large stock from a high-quality online store, the fish were indeed much healthier and disease-free.
By the way, when you do your partial water change, you need to do no more than 30%. It is also the best to have your new water ready at similar temperature as the tank water to avoid temperature shock. I have made one mistake years ago when I was getting rid of meds I used to treat the fish. A 70% partial water change with water too cold. And the fish dropped like flies within the next a few hours. Some fish did survive the ordeal. Like the 5 out of 6 corydoras. They were showing breathing heavily though.
So avoid shocking them is the first thing we need to make sure to keep them alive.
