Your fish probably went into shock as a result of the waterchange. The most likely scenario is the temperature and properties of the replacement water were too different. Next time make sure they all match or come close before you add the replacement water and things should be better for your fish. Best of Luck!
Fishkeeping is a great hobby and very rewarding to say the least, both fresh and saltwater equally. A few good tips for success: ALWAYS quarentine new arrivals for AT LEAST a month before adding them to your main, always maintain your pets accordingly, feed varied diets for top notch health in your animals and enjoy them. I wish you all the best of luck!