Heavy Breathing

[TheWindWaker]

Normally my little guy is pretty active, swimming around and 'glaring' down everything. But tonight when I looked up I noticed he was laying at an odd angle and breathing very heavily. He also didn't seem to want to swim to the top of his tank. I did a full water change thinking this could possibly be the culprit but he's still acting the same.

Housing
What size is your tank? 1 gallon
What temperature is your tank? room temperature is anywhere from 75-80
Does your tank have a filter? no
Does your tank have an air stone or other type of aeration? no
Is your tank heated? no
What tank mates does your betta fish live with? none

Food
What type of food do you feed your betta fish? betta bite pellets
How often do you feed your betta fish? in the morning

Maintenance
How often do you perform a water change? once a week
What percentage of the water do you change when you perform a water change? 100%
What type of additives do you add to the water when you perform a water change? Prime

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

Ammonia: n/a
Nitrite: n/a
Nitrate: n/a
pH: n/a
Hardness: n/a
Alkalinity: n/a

Symptoms and Treatment
How has your betta fish's appearance changed? No
How has your betta fish's behavior changed? He's now at the bottom of the tank breathing heavily and won't respond to me or mirrors.
When did you start noticing the symptoms? Just a few hours ago
Have you started treating your fish? If so, how? I changed the water as soon as I noticed his behavior
Does your fish have any history of being ill? No
How old is your fish (approximately)? I got him in June so I've had him for 9 months. Could very easily be older as the other betta I got the same day died a month ago very suddenly.