Sad Fish :'( Sick? Hungry? Bullied?

One of my two fish is having problems and I'm not sure what to do... It
seems to be having real trouble eating food off the surface of the water,
although it tries to, it sort of ends up not being able to grab it and
pushing it away. It usually has to wait until some of the food falls to
the bottom before it can eat it. Over the last couple of days, it has
taken to just resting on the bottom of the tank far more than it used to.
And there are times when it is swimming around, that it just points almost
directly down... It's when it's down on the ground that I can see that my
other fish really comes up to it, and almost looks like he's nipping at it.
At first, it used to jump up and run away, but sometimes it just sits there
and takes 3 or 4 'nips' until it moves away. The fish that is doing the
nipping seems fine. Actually, I'm going to move the sad fish out now, the
other fish doesn't seem to be leaving it alone... Hope that's the correct
thing to do...

Any advice? Poor little Freyja is too young to go!