How can you tell if your VERY VERY HEALTHY goldfish is dieing?

my fish used to float around this red plant and then nestle in a little indent in the gravel fo the night than a couple of weeks ago he just floated by the smaller green plant in the night and floated by the glass of his 20 gal. that he has all to him self during the day. he is the only fish that i own . he is about a year and a half old and has had ick , fin rot, and a blood infection in the past. i am thinking of getting another goldfish . is this a good idea ? gill appears to ether be a comet gold or a common gold. if you can tell me how to tell if hes dieing please do.