If you bought the 2.5 gallon bow in the kit, it should come with a filter... I'd put that one, but don't run the carbon in the filter cartridge (it will soak the meds right up). Clean water can go a long way in making a sick fish healthy, and it will be water for the water quality in the long run as well. I'd also look into getting a small gravel vaccum. You can get the mini one at walmart for about $4. It would work perfect on your tank. Try that (filter and water changes) and keep medicating. Simply with cleaner water it might perk up a lot more.
FWIW, your betta doesn't look like it is suffering from Heximita at all. And just for reference, is this a male betta or a female? Did it used to have long flowing fins, or were they always that short?