So I've been turning my filter on and off lately, debating if he's happier one way or the other.
The filter's pretty strong though, if he swims too close he ends up having to violently struggle to get away and the entire surface of the water is disturbed so I haven't seen any bubblenests (or maybe he just doesn't make any.) I'm not sure if it'd be better to just turn it off and do weekly water changes.

But my main question is regarding the light that came with my Eclipse tank. I love having it on because it allows me to really see him and all his beautiful colour but he seems... a little stressed with it on. I turn it off at night, but during the day I'll find him just sitting on the bottom looking sad... and when I turn off the light he'll swim around much more willingly.

Is, or could this light be bothering him? Or is this behaviour just an odd coincidence?

It probably is not bothering him. I leave mine on all day and turn the light out at night. It helps to heat the tank. If you want to teach him to adjust slowly I would put the tank uder a table lamp and leave it on and the tank light off. After a couple days of this I would leave the tank light on and leave the lamp off. That way with the lamp on and the tank light off for the first few days you are giving him inderect light.

Do you realize that turning the filter on and off kills the nitrogen cycle? I hope you are treating the tank as if it did not have a filter concerning water changes. If not, the ammonia and nitrite levels may be too high, causing him lethargy. Could you test those for us? How long have you had him? What color is the gravel? Sometimes if you have brightly colored gravel it makes the tank too bright for fish. Switching to black may help and it looks pretty also if you like the look.

What size tank and what filter do you have?

Hmm, I didn't know that thing about turning on and off the filter. He doesn't seem sick or anything, just a bit more sulky when the lights are on.

The gravel is a medium blue, I've had him about a week now.

The tank was set up the day before that but with completely cycled water and two decorations from the cycled tank plus an old media from the filter of a cycled tank.

The tank is cycled... I'm doing a 50% water change later this afternoon.

I left the light on all morning and he seems happy. I do however, currently have my filter off. It seems he's okay with having one or the other but when it's both the light and the filter running he's noticably less active.

Right now he seems perfectly fine and happy. He's eating well and swimming around and even occasionally blowing a few bubbles, to build a nest? I'm not sure but I take it as a good sign.

He will actually come over and look at my computer screen when I watch YouTube videos! lol! It is probably just a coincidence, or he just wants to investigate where the vibrations from the sound waves are coming from. But it's too cute. I love to think he's watching them with me.

I was refering to the light when I mentioned turning it on and off not the filter

Oh, I know. I was referring to Kissy's comment.