Listless Gourami...

I have a honey Gourami in a 24gal, cube, planted tank who is suddenly acting listless; just kinda sits on the bottom in a little cove area...doesn't move much, is eating much less when I feed (which seems to be the only time s/he swims around), no evidence of ich or any gill or scale problems I can see.

The tank is well established (over a year) and very steady on readings. Other occupants are 5 white cloud minnows and 3 otocinclus. No new plants or anything have been introduced. The other fish are doing well--as they have been for a year. Temp is 71, Nitrates ~20ppm, I do regular water changes, no spiked ammonia, ph about 7.8 (as it always has been). I feed Tetra flakes with occasional frozen brine shrimp and bloodworms.

Could this just be old age or ??? Should I get a kit and check something esoteric? Why is only the Gourami effected?

Thanks in advance for any tips....

Still kinda listless today. I fed some brine shrimp yesterday; ate a few of those. Flakes today and not interested (so the minnows chomped 'em all up).

0 nitrites, 0 ammonia. Temp is 71. I'm estimating between 10 and 20ppm nitrates.

I do about 30% water changes at *least* once a week; sometimes more often.

The Gourami just sits on the bottom kinda listless, or floats around *very* slowly and seems kinda spaced out.... Most of the time, it's on the bottom....

I don't know how old the fish is of course--it came from the Local Shop...so I'm assuming it was a juvenile as it grew in size a bit--it's now a bit over a year old.