Newbie - I think my fish is starving

[SpringSprite]

I have four glofish tetras, they started in a 5 gallon (I didn't know) and are now in a 20 gallon, tank is not cycled (again, I didn't know) but I have been doing water changes faithfully and today's levels were ammo -.25 nitrites .25 Nitrates 5

Three are totally fine, the 4th has not eaten in maybe a week? Is looking kind of hunched / angular on his back (not rounded like the other ones) and has a long, whitish, stringy poop? hanging from him, also for days.

I don't know what to do! So much conflicting advice about fish illness, all I know for sure is that he is not eating. I don't know what foods to temp fish with, what should I go out and buy for this guy tomorrow to tempt him? He's got to be dying, he just won't eat

I'll get any medicine / food you recommend, I just don't want to do the wrong thing and possibly compound the problem. He kind of just hangs around the bottom of the tank and is really slow...

He was definitely fine when I got him. He doesn't seem to be in serious distress yet, but I am at a loss - it's so sad to see. I would euthanize him via the clove oil method I've read about if necessary but obviously don't want to be hasty, yet also don't want him to suffer.

Any help appreciated.

 

[SpringSprite]

bump.

He has not eaten. I bought some API general cure, which I hope will treat the "wasting disease" and possibly some internal parasites which the stringy poop might indicate.

I think there might be carbon in my filter, tho, it's a bio bag. I am waiting for a Aqua Clear 50, but wanted to keep these filter bags in for the BB. So, in having just dumped the medicine in the tank, are the filter bags rendering it useless?

I have NO idea where to go from here. Which is more harmful, the non cycled tank, or the potential carbon in the filter bags