my fish gloria

I have two goldfish in a 3 gallon tank. One, named Gill, has grown HUGE
and will feed on everything in sight. The other one, named Gloria, has
stayed almost the exact same size as when I first got them together
about 4 or 5 months ago. Gloria isn't interested in eating- she never
comes to the top of the tank for food like all fish know to do, but
will eat if food seems to float past her. I've had a hard time
maintaining the ph and ammonia levels (they fluctuate so much and never
stay steady!) and I think it has affected her... she's now hanging out
at the top of the tank with drooping fins and sometimes is upside down.
I can't see any skin problems and she isn't ballooning from dropsy. I
also never see her poop, and have tried feeding her yogurt as a trick
for constipation, but that has been unsuccessful in increasing her
appetite.

Suggestions are appreciated! Do I just have a hopelessly sick fish?
I've never really seen her happy, and the other one is doing almost too
well.

Thanks-
Antoinette