Is My Fish Ill? Please Help

A few things spring to mind.

You say you have plastic plants - do they have sharp edges? These can cut a betta's fins.

Tankmates - cories should be OK and copper harlequins (are they Trigonostigma hengeli?) are usually OK with bettas but there is the possibility they are nipping his fins. Or they could just be stressing the betta as they are faster swimming fish that he is.

The filter. You say you've changed the filter twice. Do you mean the whole thing or just the media? If it's the whole thing, did you keep the media from the old filter and put it in the new? If you changed the media, with or without the casing, you would be doing a fish-in cycle every time you removed old media, and the resulting ammonia could be burning the betta's long fins.
Are you monitoring the ammonia and nitrite levels if you have been changing filters?
How strong is the water flow from the filter? Some bettas struggle if the water flow is too fast.

Temperature - 76.5F is 24.7C and that is a bit on the cool side for bettas. They like it around 26-27C or 78-80F.


Personally, I would set up a tank just for the betta. Something 20 to 25 litres is perfect, with a filter that has a slow flow, and a heater set to 27C/80F. Silk or live plants. And places for him to hide in and swim through. Plenty water changes (especially if you don't cycle the filter before putting him in) and even if the filter is cycled, still plenty water changes till his fins heal.

Don't use the ammonia removal liquid. You need to do water changes to keep the ammonia level below 0.25. Since you changed the filter you are now doing a fish-in cycle. You need to measure your ammonia and nitrite levels every day and do a water change if you see either or both above 0.25. Once they stay at zero by themselves, you can do weekly water changes.
With an ammonia reading of 1, you need to do a 75% water change to get it to 0.25, but then it will creep up past this level, so either a 90% water change or 2 x 75% changes an hour apart are needed for a 1ppm ammonia. Having any ammonia, let alone levels over 0.25, will prevent the betta's fins from healing. And water changes are also the best way to help his fins heal even without any ammonia or nitrite in the water.

I would also check your plastic plants for sharp edges. If they have any, it would be a good idea to swap them for silk plants or more live plants.