Fin rot that won't go away

[MeB]

Housing
What size is your tank?
1 gallon
What temperature is your tank?
70 degrees?
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?
variety of bloodworms, pellets, flakes
How often do you feed your betta fish?
twice a day, a small amount

Maintenance
How often do you perform a water change?
whenever i have time...try to about every 2 weeks
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" dechlorinator

Water Parameters:
Have you tested your water? If so, what are the following parameters?
Haven't tested it yet, can't afford a tester
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed?
fins half gone
How has your betta fish's behavior changed?
see background info
When did you start noticing the symptoms?
see background info
Have you started treating your fish? If so, how?
yes, with bettafix, i think
Does your fish have any history of being ill?
see background info
How old is your fish (approximately)?
1 and 1/2 years i think



BACKGROUND
My little sister got a veiltail male betta from petsmart about a year ago, he was very healthy, beautiful long fins, smooth scales with no problems, gills in good shape. I didn't pay much attention to him, I have a fish and a cat of my own, but about two weeks ago I noticed he looked very sick.
He was very skinny, starving, and his fins were definitely rotting off, almost completely gone.
I took him into my care, changed his water, added BettaFix as the bottle said, and started feeding him. For a few days I was worried he wouldn't make it, but now he is looking almost as good as when she first got him. I moved his tank onto the shelf next to my betta's, and they flare at each other quite a bit.
He has filled out and lost his underfed look, but his fins haven't come back. They grew about half-way back, but then stopped. I was wondering if the damage was permanent, or if there is any way I can get the rest of his fins back. The edges are brown, which is not their normal colour.

[Oldfishlady]

I would start by making some water changes 100% daily for several days and get the water temp up to at least 76F

Due to poor conditions and nutrition from your sister it may take a while to get re-growth of the fins and boost his immune response.......improve water quality and nutrition

A 10 day leave in salt treatment could be helpful once the water quality and nutrition are improved

Use Aquarium salt 1tsp/gal along with 100% daily water changes while in QT for 10 days
Adding oak or IAL to the tank and steep in some pre-mix salt water 1gal jug would be helpful as well-this will also help make correct dosage and water changes easier.
Feed a high protein varied died small amounts several times a day

Once treatment is completed start a regular water change schedule in the 1g unfiltered container to-100% 3 times a week.

You should see fin re-growth within a few day to several weeks depending on how bad he is.

[MeB]

I'll try with the aquarium salt, I don't know when I can get it though. I will try to change the water daily, but i may not be able to due to schoolwork overload. Thank you for the advice, I've had bettas with fin rot before, but they've all gotten better quickly with some BettaFix, so I wasn't sure.