Lathergic Goldfish Sitting on it's Face

[Luckeystiff]

I purchased a small Ryukin goldfish this friday, and ever since I bought the fish, it has spent all day at the bottom of the tank tilted slightly forward so it appears to be sitting on its face. The tank is 30-35 gallons and the params as of now are:
Ammonia 0
Nitrite 0
Nitrate 5ish

The fish came from an unheated guppy tank so I wonder if the lack of movement has sapped its energy. There are no visible issues on the fish, aside from the lack of movement. The fish only exits from the cave to eat, and only eats sinking foods. The tank is outside, and goes from 20°C on hot and sunny days, to 15°C on cold nights. It is filtered basically by a 100gph pump wrapped in filter floss cycled from my larger community tank, this provides mechanical and biological filtration. I plan on eventually adding a second fancy goldfish such as an Oranda or another Ryukin. Would tank mates help it's energy? I did a 60% water change yesterday to clean out the environment, and the water is now almost perfect.

I have attached the seller's image of the fish posted on thursday.

Does anyone have any idea of how I could get the energy back in this fish?

 

[Delapool]

Do you think it is a balance problem from around the stomach? Hard to tell if it looks a little bloated or not.

 

[Luckeystiff]

He was swimming fine inside the bag, but he is also starting to come out more. I think he might just be getting used to the outdoors.

 

[Delapool]

Fingers crossed! Sounds great news and nice photo as well above.

 

[Luckeystiff]

Thank you, I think it's also helping that the water is warming up. I'll still wait until he is comfortable before I add another fish.