Sick weather loach help please!

[CheeseSticklover99]

I have a 3 year old weather loach who was fine one day and the next he's laying on his side and not swimming really at all. I thought maybe he was constipated but he pooped so guess not. He wants to eat, when food is next to his mouth he will excitedly eat it, but won't swim around to get the rest of it so I am handfeeding him, his appetite hasn't changed.

No signs of any illness/disease/ parasites on the outside. Breathing normal. Just laying on his side and won't move. This has been going on for two days now. Put him in quarantine yesterday. I have two other weather loaches from the same tank and they are doing fine.

The three of them share a 75 gallon tank that's 4 ft x 2 ft x 2 ft.

Cold water tank
PH 7.2
Ammonia 0
Nitrites 0
Nitrates 20 ppm

Laying on their sides, upside down, in a tree, wherever, is normal for these guys but...
This guy in quarantine looks like he's struggling to stay right side up sometimes and not swimming at all. Laying on his side for 2 straight days. I'm so worried! Any thoughts or comments are much appreciated. Thanks everyone.