Another unwell balloon molly

I've got four small white balloon mollies in a ten-gallon planted tank
at 79 degrees F which has been running for a few weeks now. I used a
good percentage of water from our main tank to start this one up, which
I think may have helped me so far avoid an ammonia or nitrite spike.
Both ammonia and nitrite levels are virtually zero, and I've done a
weekly 10% water change this afternoon. But one of the mollies is quite
lethargic- spending a lot of time near the bottom in one corner or
another. She (he?) becomes briefly active at feeding time but then goes
down to the bottom to rest again. There is no visible sign of disease
that I can see- bright white colour, no signs of ich or injury. The
other three are fine.

So what can I do to help this molly?

Many thanks,
Ian.