My betta fish........is he sick/underfed?

I just got my betta fish 3 days ago, and I have a one gallon tank with a filter system and water heater, and the water is clear and seems fine (but I am not sure about pH levels, the water is tap water that has already been treated with water conditioner for the betta [I treated the water with the fish conditioner about a week ago] and I haven't added anything else in the water). My betta fish's fins aren't as fanned out, but they don't look like someone took a bite out of them or anything like that. Its colors seem to have gotten darker and less vibrant, and he seems to be not swimming around as much, and he sinks sometimes to the bottom of the tank when he doesn't swim around and just stays in one place. The angle at which he swims at also is different; his head is more tilted upwards a little. I've fed him only a little bit of food every day for the past 3 days I've had him. Does this mean he's underfed or sick or what???????