[JouteiMike]

Sounds like your fish are stressed out, and your conditions are pointing me in one direction...poor water quality.

Do you have a liquid test kit? If not, get one and test your water immediately, and post the results for ammonia, nitrite, and nitrate. How big is the tank? How many fish and what kinds are in the tank? What is the pH and temperature? How long has the tank been set up? More information is needed on your tank.

Columnaris, or mouth fungus, is what the guppy had. The fish should have been promtpy removed, because this disease is highly contageous. As for the comet, are you talking about the species of Goldfish? That itself could lead to problems. Sounds like it had fin rot, and/or symptoms from other diseases like columnaris. Sounds like an extreme case when you say the fins have disappeared, so that means the may beyond the point of repair.

 

[JouteiMike]

The comet alone will need a much bigger aquarium. Remember, these fish are more suitable to live in ponds than aquariums, they need at least 75-90 gallons, and can get up to 12". And as mentioned, it is a coldwater fish, your tank is too warm for it.

I'm guessing since your tank is newly set up that is still cycling. This can be very stressful and deadly for the fish due to the high levels of ammonia and nitrites. For now, keep just the goldfish in the spare tank, and take the heater out if you have one in there...I'm hoping you are going to return the Comet to a quality fish store unless you can obtain a much larger aquarium.