I have a freshwater tank with four Australian Rainbowfish. Recently,
one of them has had trouble swimming. He can swim with the group, but
normally ends up retreating to a place where a plant is near a wall,
so he can sort of balance himself between them.

In all cases, his head points upward somewhat. When he's in that
resting place, he is more than 45 degrees from horizontal. His
pectoral fins and tail are constantly moving to help him remain
afloat. When he rests those, he begins to sink.

I don't see anything physically wrong, and the other rainbows seem OK.
I did notice the temp was a little high (83), so I turned down the
heater. FWIW, that resting spot he hangs out in is very near the
heater (but also near the flow from the filter, so I don't know what
the local temp is right there).

Any ideas what this might be and what I can do?

Thanks.