I posted some of this earlier in the general betta care forum but now that I'm more convinced it's an illness, I'm recapping/updating here to get some additional opinions...
What size is your tank? 2.5G
What temperature is your tank? 78 degrees
Does your tank have a filter? No
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None
What type of food do you feed your betta fish? Hikari Bio-Gold pellets
How often do you feed your betta fish? 2x/day, 2 pellets each time
How often do you perform a water change? 2x/week
What percentage of the water do you change when you perform a water change? 50/100
What type of additives do you add to the water when you perform a water change? API Stress Coat
Have you tested your water? No
If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed? He looks "smaller" somehow - maybe he's not extending his fins as fully? Maybe a little paler too but hard to tell; his color has always been quite light. No apparent discolorations (no white spots, no blackness/redness of fins, etc)
How has your betta fish's behavior changed? Much more lethargic than usual, spending lots of time resting in mug, bottom of tank, or surface of water. Occasional bursts of energy where he swims around quite a bit, but resting majority of the time. Lateral fins are clamped, except for when he's swimming. He is still eating normally and is still pooping regularly but a couple of times he has been so still and unresponsive that I thought he was dead.
When did you start noticing the symptoms? Yesterday
Have you started treating your fish? If so, how? So far all I have done is a 50% water change yesterday morning when I first noticed he seemed ill. I was worried to do a 100% change in case it was stress-related I did not want to stress him further.
Does your fish have any history of being ill? No
How old is your fish (approximately)? Unsure; I got him about 2-3 months ago.
Thank you for any advice or suggestions!