Devin wrote:

I have a fish that seems to be lazy lately. He sits on the bottom of
the tank, just staring around, unless he thinks it's time to be fed,
and then he's very active and swimming.

No problems with water parameters (good PH, no ammonia or nitrites),
good
temp, etc. The other 3 fish (4 fish in a 40 gallon tank) are all
fine. The
fish in question looks healthy, just lazy. It's been going on for a
few days.

Any ideas?

50% water change with treated, temp matched water. Sometimes water gets
bad without changing your measurables.

This is a change in behavior so Singer's post is a bit off.

-D
--
"There is nothing so strong as gentleness, and there is nothing so
gentle as real strength." St. Francis de Sales