[Aluyasha]

So my DTHM, Julius, whom I have had for about a month now, has been doing great. Eating like a king, flaring at my cats, and always swimming around and messing with things. He was in perfect health only hours ago.
Well I got home from the petstore at about 9pm (it is 10:30pm as I type this) and I went to feed him and he is lying vertically, breathing hard, and unresponsive. He did not even notice me, or the food.
His appearance still looks normal, nothing has really changed in his tank. His water temp is at 80 degrees but has been steady.
I have no idea what is wrong and am terrified he is on death's door. Any opinions welcomed.

 

[Aluyasha]

Slight update:
Well he is getting quickly worse. It has only been an hour since I made this thread and he looks barely alive.
I did a water change and gave him some aq salt, and some stress zyme. When the water was still moving from the water change he was floating around stiff. Now he is laying upside down on the gravel, barely breathing.
Unfortunately I will be shocked if he makes it through the night...

 

[Aluyasha]

...Well, he has passed away. He went so fast I still cannot believe it...
I tried to save him but it seems nothing could. He was fine just a few hours ago.
I am so sorry Julius.

 

[Romad]

I'm so sorry about Julius He was a beautiful boy.

Did you test your water paramaters when you saw that he was off? Could it be that something was added to your city or town water (assuming that you don't have a well)?

It's frustrating when something isn't obvious and you lose a fish but you may never know what happened. You haven't had him for very long so he could very well have had issues that just manifested themselves now.