Thanks for all the helpful feedback so far! Apologies I didn't have a chance to fill out the form sooner. Since yesterday, the dark spot on the top of his caudal tail appears to have some decay. I've attached clearer photos of the spot in question from yesterday and today.
Housing:
How many gallons is your tank? 3 gallon
Does it have a filter? Yes
Does it have a heater? Yes
What temperature is your tank? 80 degrees
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? Cobalt Aquatics
Do you feed flakes or pellets? Pellets
Freeze-dried? No
How often do you feed your Betta? How much? 2x per day, 6-7 pellets total
Maintenance:
Before your Betta became ill how often did you perform a water change? Every two weeks
What percentage of water did you change? 50%
What is the source of your water? Top Fin pre-conditioned aquatic water
Do you vacuum the substrate or just dip out water? Vacuum
What additives do you use other than conditioner? What brand of conditioner? API Stress Coat
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: Need to get a separate test kit for this
Nitrite: 0
Nitrate: 0
pH: 7.0
Hardness (GH): 60
Alkalinity (KH): 40
Symptoms and Treatment:
When did you first notice the symptoms? Around a month ago
How has your Betta’s appearance changed? A blue stripe appeared at the top of his caudal fin. Since it appeared, it has gotten darker and looks swollen. It appears to have also started decaying. I've noticed his right eye looks like he may have a cataract as well.
How has your Betta’s behavior changed? His behavior has not changed much. He likes to lounge but is still active. He's recently started resting against the filter, which I'm wondering if this may be causing an issue.
Is your Betta still eating? Yes
Have you started treating your Betta? If so, how? I put him in a hospital tank yesterday with a small amount of aquarium salt and his heater.
Does your Betta have any history of being ill? He was sick last year with what I believed to be swim bladder, but has not had any issues since then.
How long have you owned your Betta? About a year and a half
Was he or she ill or suffering some sort of damage when purchased? No, I don't believe so.