Should I be concerned?

[xswornxoffxjello]

Thor isn't himself. I can't put my finger on what's wrong, exactly; he just seems really tired. Usually he's always flaring and swimming around, but now he's just lying there near the bottom. I tried to stir him up a little to see how he'd react, and he swam all right but needed to stop, like he was out of breath. Usually he's very active and healthy. He's in a divided 10 gallon, heated at 78 degrees, filtered, uncycled. The last water change was 50% today, but he was acting strangely before that.

[Welsh]

Maybe he is bored of the scenery, have you tried re-arranging his tank or adding something new? maybe he just needs something to rest on, do you have ornaments in the tank?

How strong is your filter? a harsh current from a filter can really be stressful for a fish.