Weird discoloration on lethargic betta! Please help!

I purchased my betta a few months ago and he has been in a heated 5 gallon, with stagnant water. I did regular water changes and have tested water samples. All came back with non-threatening results. After noticing my betta becoming more lathargic I also noticed fade white, almost gray...I'm not sure. They're not spots or patches. Just like skin discoloration along his sides and near his gills. Could this be ammonia poisoning? I just got a filter set up and have added aquarium salt after the 50% water change. His ammonia levels are 0ppm though.

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What size is your tank? 5 gallons
What temperature is your tank? 78 degrees
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? Yes
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? Pellets
How often do you feed your betta fish? Once a day

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? 25%
What type of additives do you add to the water when you perform a water change? Water conditioner, aquarium salt, stress zyme

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

Ammonia: 0ppm
Nitrite: 0ppm
Nitrate: N/A
Hardness: N/A
Alkalinity: N/A

Symptoms and Treatment
How has your betta fish's appearance changed? Some of his skin has turned a pale white, but it is not a patch or spot. Just discoloration.
How has your betta fish's behavior changed? He is a lot more lethargic, laying on the gravel and taking longer to respond to me. I thought he was dead this morning.
When did you start noticing the symptoms? 2 weeks ago
Have you started treating your fish? If so, how? Changing the water and adding aquarium salt
Does your fish have any history of being ill? No
How old is your fish (approximately)? 1 year old