Your comet has Slime or Velvet disease, it will cause his to be lethargic. He probably has a slightly milky discolouration on his body and fins, like there is a 'film' covering him.
This is a VERY common disease and is caused generally from poor water quality. Your water could be poor from over feeding your fish, this is generally the reason for poor water quality.
The best thing you can do now is to make sure the water is changed and clean, add a tiny bit of salt into the water (table or aquarium salt) and/or go to the petshop and buy some smile and/or velvet medication. Also make sure your water is well oxygenated, do you have a filter?
Best of luck!!!