Why is his fin dissapearing?!

[MikiMaki]

Housing
What size is your tank? 10 gallon
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? just the hob filter
Is your tank heated? I don't have it on as the temps have been too high lately
What tank mates does your betta fish live with? A female on the other side of the divider

Food
What type of food do you feed your betta fish? Top Fin Betta Bites
How often do you feed your betta fish? 4 pellets twice a day or 6 once a day a blood worm or two once a week and fasted once a week

Maintenance
How often do you perform a water change? Every other day (except now with treatment)
What percentage of the water do you change when you perform a water change? 10%
What type of additives do you add to the water when you perform a water change? Stress Coat

Water Parameters:
Have you tested your water? If so, what are the following parameters? All good when I took them in to be tested when this started.

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

Symptoms and Treatment
How has your betta fish's appearance changed? He's missing 2/3rds of his caudal fin and shredding the others.
How has your betta fish's behavior changed? Not at all, he's even started building nest again (he hasn't since he got moved into the big tank)
When did you start noticing the symptoms? Peterson took a chunk out of his tail a few weeks ago and it had been getting better til last week.
Have you started treating your fish? If so, how? AQ salt and Trisulfa
Does your fish have any history of being ill? He had fin rot once when he was in a small tank (my bad =( )
How old is your fish (approximately)? 2 years.

This was earlier this week and now it's a good half an inch or more shorter, there's no black around the edges, he has to be really flexible to be chewing it off, there is nothing in his tank that he could be ripping it on (took out everything but three silk plants including substrate just in case) there is always the clear new growth going on one day and then the next he's got even less tail! He's such a problem child!

[Creat]

It looks like finrot but thats just my idea :/ how are you treating him for it? If he hasnt already i would qt him from the female so she doesnt catch anything

[MikiMaki]

She's actually in a qt tank because the trisulfa I'm using to treat it is a tablet for a ten gallon, which would drive me nuts to try and divide the tablet into a 2 gallon dose. So the trisulfa and the aq salt are what I'm using to treat. The edges of his fins have regrowth on them almost constantly and theres no black or red on the edges, no debris on the bottom of the tank from it. But you could be right, after tomorrow night he will be done with his dose of trisulfa and I will do a 50% water change and then 10% a day after that, or should I up that?

[Tikibirds]

I use stress coat which has aloe that helps with fin regrowth. It can also be used as water conditioner.

[Creat]

Glad he is doing better! You probably might want to up the 10 to a 25 also did the meds kill the bacteria in the tank be careful or the tank might recycle on you and that would not be fun :(

[MikiMaki]

I seriously didn't think of that. Crap, I will take a sample to the store today and see what harm that did and pick up some more test strips. He's got pretty significant regrowth now and it's been two days since anymore damage has been done.

I've not done a huge water change before, more then twenty percent. I usually keep premade 1 gallon jugs of water + conditioner made up and set out over a few days cause I don't trust the dechlorinator to work immediately. So I could take the big guy out, change water with straight tap water into tank and then add stress coat straight to tank and swirl and it should be perfectly safe to put the big guy back in? I know that sounds so novice but I don't trust the chlorine lol.

[Creat]

Thats the real challenge with treating cycled tanks the meds can kill off all te hard work of cycling it, sometimes... I have a 5 gallon bucket that I use so I can take out 25% of my water (thats how much I take out per week on a cycled tank) and I can refil that from the tap and add my dechlorinator along with other stuff straight in (usually aq salt, maybe IAL extract, etc. ) then pour whats in the bucket in the tank. For next time if you have to treat him take out the filter pad or something that has bacteria on it and keep that like in a separate container you could also ask the petstore for some of their filter pad if thay let you and if you trust their fish to reseed the tank with... dont want anymore diseases :)