Baby betta lethargic at top of tank

Housing:
How many gallons is your tank? 2g (for now)
Does it have a filter? yes
Does it have a heater? yes
What temperature is your tank? 78F
Does your tank have an air stone or other type of aeration? no
Does your Betta have tank mates? What kind? no

Food:
What food brand do you use? aqueon
Do you feed flakes or pellets? pellets
Freeze-dried? no
How often do you feed your Betta? How much? 2-3 pellets once a day

Maintenance:
Before your Betta became ill how often did you perform a water change? just started
What percentage of water did you change? just started tank
What is the source of your water? store bought betta water because the well water is not good
Do you vacuum the substrate or just dip out water?
What additives do you use other than conditioner? What brand of conditioner? don't have it on hand not home atm.

Water Parameters:
What are your water parameters? Please give exact numbers. If tested by pet store please get exact numbers. "Fine" or "Safe" won't help us help you. Important: Test your water before the regular water change; not after one.

Ammonia: 0
Nitrite:0
Nitrate:0
pH: 8
Hardness (GH):200
Alkalinity (KH):300

Symptoms and Treatment:
When did you first notice the symptoms? yesterday
How has your Betta’s appearance changed?N/A
How has your Betta’s behavior changed? lethargic not gasping
Is your Betta still eating? one pellet
Have you started treating your Betta? If so, how? added PH balance an hour ago
Does your Betta have any history of being ill? N/A
How long have you owned your Betta? 4 days
Was he or she ill or suffering some sort of damage when purchased? Was gifted to me but 30 degrees outside during hour travel home

Bear in mind I'm not professional so I can only equate your situation with what I observed in mine. I first had him in a 3L (<1 Gallon) temporary home until my cousin brought the 20 gallon (I ended up having to buy a 10 gallon the next day cause issues... lol) In this temporary small 'tank' he just chilled on the bottom. Which had my mind racing that his finnage was so heavy he couldn't swim much without tiring out (another reason I couldn't use the 20g - cause it was deeper...) Anyway, I put him in the 10g after it reached the appropriate temperature. His lethargy was instantly cured - he swam around up and down didn't really sit still (so obviously his ball gown of a tail wasn't too heavy) - After my 2nd cleaning I added a bunch of decorations to the tank - probably about 50% of it was taken up by the decor/fake plants and etc. - and he began chillin' out more again. So knowing that my decor, heater, and filter took up a good 5g of tank space in a 10g tank - I'm going to guess that your apparatuses and decor are taking up close to 50% of your tank, and so your fish just doesn't have much room to really swim or explore. (After all, we have a completely different swim quality between a bath tub, above ground pool, and public pools - it's pretty much the same for fish )

Check facebook market place for stands/tanks with stands. I saw a sweet coffee table aquarium I was interested in (if only I wasn't on the 2nd floor!) your boo probably just needs more space to just keep swimming swimming swimming swiiimmminnnGGGG.

Only thing I really see concerning is you potentially over conditioned your water;

"What is the source of your water? store bought betta water because the well water is not good
What additives do you use other than conditioner? What brand of conditioner? don't have it on hand not home atm."

^ you used a conditioned water, and added a conditioner to it?