Crooked spine can be many things from fish tb, internal parasites, injury, birth defect, vitamin defiency.
What do you feed your fish?
Signs of internal parasites are.
Long stringy white poo or clear mucas poo.
Worms prutruding from the anus.
Being skinny or bloated.
Bent spine.
Fish will swim on there sides sometimes.
Sunken in belly,.
Enlarged anus to red inflamed anus.
If the fish looks like its suffering I would end its misery.
The diet could do with improving, frozen foods like daphnia and brime strimp, green veg.
I would also issolate the fish.
Has she turned black anywhere. Has the iris gone black. Or eyes bulging out.
At a guess its just a birth defect. Don't euthanise!! Its not uncommon for bent spines to bend back and heal ive seen it happen in fry Well done for taking the precautions of separating him though
Finlay
I have a male guppy with a bent spine, it bends from the back of his head, he was born like it but he has no problems swimming or feeding, he is about 6 months old now and doing well