My betta Jack Sparrow is usually pretty lively and acts like he hasn't eaten in years when I feed him. I have had him for about 6 weeks and he has a heater with temp at 79 to 81, he has a 3 gallon tank, he has a hiding place, a plant, and two ornaments, and I do two %50 water changes a week and one %100 change a month. Water prams are fine and I just did a water change yesterday.
The last few days he has been a lot less playful and it took me a good 15 minutes of mirror time to get him to flare. He only eats one pellet a day, which I know is ok it's just a drastic difference from three to five pellets a day. He does not have physical signs of illness like bloat or any other disease but he is acting a lot like when he did before I got him a heater.