Guppy staying in corner of tank

[VanillaPepper]

I've got a 5gallon tank with two male guppies and a mystery snail, it does have a filter, air stone, and hiding spots.
The tank has been at 78° the whole time.
My platinum guppy was very active, moving about up and down and all around. But the last two days he's just hung out in the corner beside the filter. He's still eating fine, my other guppy will come and hang around him but he doesn't stay there like this one does.
I'm not sure what's wrong? Any help would be appreciated! Thanks.

 

[Aiken Drum]

Any other signs of illness or injury? Is the fish otherwise swimming alright?

 

[VanillaPepper]

There doesn't seem to be any signs of sickness. He moves around within that corner but he won't come out away from it. I was wondering if he might not like the flow of my air stone.