My poor fish

...ok...so yesterday I change the water in both of my fish tanks......well....one of my tanks. Well one of my fish (manganeus) was basically dying......oh man! I took him out and tried to keep him active.......wasnt working. I moved him around in the tank by hand....still no go. I finally gave up....didnt want to but then I figured I couldnt save him. So I figure if he's gonna die......damn....had him for years now (4) I came back to the tank and noticed he was light colored and tail curled him on his side in the corner of the tank.................that was it.. I moved him and poked him, he just laid there. =( I'll post pics as soon as my batteries charge to my digital camera. So I put him in a cardboard box, small one. And all of a sudden while standing there he started to move his tail! ! ! I was like HOLY SH*#T!! put him back in the tank and when I came home later that night he was back to normal! Can you believe that? I have no explanation for that........cant beleive it.....even today he is perfectly normal. I shall treat him to some feeder guppies tomorrow. Infact I will treat all my pets to something nice tomorrow........my trantula i'll get some crickets..my iguana I will get some gooodies, and my turtle i'll give some gold fish to eat. And my other fish will get some feeder gold fish!