HELP! Betta fish staying at bottom of the tank

Hi all, I’ve noticed my betta fish is staying at the bottom of my fish tank a lot and only coming up for air, she sometimes comes up once daily for food but that’s it. I have also noticed her top fin is rather droopy and some red has appeared on her head which I have attempted to treat with Melafix but it does not appear to have fixed anything. I researched that she may have over eaten hence why staying at the bottom of the tank so I’ve minimised the amount of food I’m feeding the whole tank. I’m doing very frequent changes (50%) of the tank and the PH and nitrate levels are both fine, temperature is also in good range. There are lots of live plants in the tank and sand at the bottom too. She does have other fish in with her but none that are new/she shows distress over. If I could have any suggestions as to what I could do next to help that would be great as I am incredibly worried.

Need better pictures and a 1 minute video of the fish if possible.
Upload videos to YouTube, then copy & paste the link here.

How long has the fish been like this?

How long has the tank been set up for?
What is the ammonia, nitrite, nitrate and pH of the water?

How often do you do water changes and how much do you change?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate the new water before adding it to the tank?

What sort of filter is on the tank?
How often and how do you clean the filter?