I would only imagine that would be an issue if you plop your fish right away into 100% new water. Which I would not recommend. I regularly do 50% water changes with no issue, because they still have 50% of their original water, at the correct balance in regards to dissolved gasses. Plus, if your filter is running, it will regulate very quickly. However I live in south florida, and our water is probably near 80 degrees most of the year right out of the tap.