Agree. Get him out of the meds.
When you redecorated the tank did you add anything new to it? Did you rinse anything new really well before adding it?
Here's how to do large water changes without shocking your fish:
To do a water change, use a little cup like a plastic solo cup - this cup must be only for him and have never been used with soap or other chems. Scoop him up in this cup (keep him in the cup about 1/4 full of water - it doesn't need to be much because he won't be in it for long) and leave him in the cup while you change his water. To do the 50% use a turkey baster - dedicated only to him that has never seen soap or chems - and drag it through the gravel and try to suck as much of the poop out as possible, in addition to 50% of the water. Use a thermometer under the running tap to get it to be the same temp as the water that is normally in his tank. When the thermometer says the flowing tap is the right temp, fill back up his tank. At this point, add the conditioner (dose for how much water you change - if you change half the water you add half gallon worth of conditioner, If you do a 100% water change dose for the full gallon change). Float his plastic cup with him in it in the new water. Slowly add a couple tablespoons of the new water into his cup every 10 minutes for at least an hour. Finally, dump him in gently but try to get as little of the old cup water back into the tank as possible. When you do the weekly 100% you will do mostly the same thing except empty his tank fully and rinse everything in it very well under warm water but never use soaps or chemicals. Once it's fully cleaned/rinsed you can refill it and repeat the cup/acclimate phase.
What type of food do you feed your betta fish? omega one betta buffet and the occasional blood worm
How often do you feed your betta fish? 2 to 3 pellets 2 or 3times day
Are you saying you feed 2 to 3 pellets total or 2-3 x 2 or x3? That's up to 9 pellets a day and WAY too many. He's fine on 3-4 total, and one fast day a week where you feed him nothing. Blood worms are fine treats once a week but if they're freeze dried just make sure you give them a through 10 min soaking or so in some tank water in a cup to make sure they don't cause constipation.
Have you seen him poop recently? What does it look like?
Your water change looks good.. I can't imagine fin rot would happen in a tank that always got that many water changes.. did you always do that many and are you sure he's not just biting?
In a filtered and fully cycled tank of that size you are doing way too many changes. One weekly change of 50-75% should be sufficient if it is fully cycled - 0 ammonia, 0 nitrites and some nitrates present. If it is not then you need to be using drops kit for ammonia and nitrite and nitrate - testing daily and making a partial water change any time you see .25ppm ammonia or nitrite: http://www.bettafish.com/showthread.php?t=47838
Once it's fully cycled you will see nitrates and to remove those you do your regular weekly change. As long as they stay under 20 you're good.
A good heater will keep your tank a constant temp and not allow it to fluctuate 2 degrees over and over. You should get another one if your temps are dipping around like that. You also need to be using an in tank thermometer to know and not rely on the heater. In something that big a 25w heater from Jager or Marineland Visitherm would work. 78-79 is fine. 76 is chilly.