Clown fish lazy or sick???

I have a 10 gallon tank with two clown fish, several snails, 5 hermit
crabs, and a cleaner shrimp. One of the fish has started lying
motionless on the bottom in the corner. If someone comes near the
tank, it will get up and come to the front of the tank. It is still
eating well, and is not showing any other strange behavior. I see no
external signs of disease. I have had the fish for about 2 years. The
only things that I have added in the last two months are two snails.
The water parameters are as follows: SG - 1.024, ammonia - 0, nitrite
- 0, nitrate - 5, ph - 8.4, kh-10, Ca- 450-500, temperature - 76-78.
These have changed little for the past year or more. I change 2
gallons of water every other week. Any idea what could be causing this
behavior?