First of all, you should be careful to put too cold water in, it'll shock the other fish as well

Try to get the new water to be almost the same as the main tank! For starters you can use a simple glass thermometer for that, they're super cheap, but once you'll have done it a few times, you'll be able to feel the correct temp

It would be ideal to have the water only 1-2 degrees colder/warmer than the tank water
