lethargic betta who can't swim

Housing
What size is your tank? 10 gal
What temperature is your tank? 78
Does your tank have a filter? yes
Does your tank have an air stone or other type of aeration?
Is your tank heated? yes
What tank mates does your betta fish live with? none

Food
What type of food do you feed your betta fish? flakes
How often do you feed your betta fish? once per day

Maintenance
How often do you perform a water change? weekly
What percentage of the water do you change when you perform a water change? 50%
What type of additives do you add to the water when you perform a water change? conditioner

Water Parameters:
Have you tested your water? If so, what are the following parameters? n/a

Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? n/a
How has your betta fish's behavior changed? he lays on the bottom or on top of his little bridge, seems to have difficulty swimming to the surface, does nto swim around well
When did you start noticing the symptoms? one week
Have you started treating your fish? If so, how? changed water, stopped feeding, increased water temperature, drained water from tank to make it easier for him to surface
Does your fish have any history of being ill? no
How old is your fish (approximately)? one year

My betta has been acting strangely. He does not swim around and just lays at the bottom of the tank or on the little bridge we have for him. He can not swim well and struggles to surface. He will swim really fast try and feed or get air but will not swim around the tank or interact as normal. He does not have pineconing or any noticeable bloating. We have tried to feed him a pea but they sink and he is uninterested. Not sure what to do next!