Asap Help With Goldfish!

[samxx]

Hello

I have a comet goldfish, he is about 6 years old . who is living in a 24 gallon tank with his comet goldfish tank mate, one of the goldfish who is orange & white has started to get gold/ yellow tail & the fins , he is not acting like he normally does & after I changed the water yesterday he got very weak & just lay on his back for hours until he started swimming again but it looked died when it was on his back , He is fine now swimming around like normally but the gold/yellow is getting more & more each day. I have put some fin rot treatment in the water. Can you help me? What is wrong with him?

 

[black angel]

Need a good pic of him to see what you mean by the colours.
Also the tank is too small for the fish. Do you test the water at all?
How often and how mucha re you changing the water and what filter are you using.

There is a disease called velvet which gives a gold sheen but need you to get a pic to verify.