I just did a water change and my fish died?

Well it is probably that the fish became very stressed due to its age. When you handle fish in changing the water, it is good to be careful. I product that I highly recommend and use is STRESS COAT. You can get this at Petsmart or Petco and it immediately calms your fish down. I use this and in about 15 minutes they swim like normal. Oh, and DO NOT WORRY about your fish dying that much. Most people have had fish die in water changes (myself included...countless times). So just put in stress coat with your normal conditioner and the fish should be fine. It's cheap (like $4) and you only use 1 teaspoon per 10 gal. which is good.