Mild Betta Fin Rot In Community Tank

Hello,
I have a little problem regarding my betta recently the fish's fins have started to wear away a little and I don't know why and I also don't know how to treat it.

The tank perimeters are good and nitrates are low. I have proper filtration and lots of plants and no aggressive community fish including pygmy cories and ember tetras (THE TANK IS 15 GALLONS). I clean 50% of the tank as well and clean the gravel weakly.

I was wondering how do I cure this as I don't have a separate tank and I can't treat the fins with aquarium salt as it's not great for corys...

How do I treat the betta and is there anything I can do to reverse the damage?

Thank you,

 

The best treatment for finrot is very clean water. I usually recommend doing 50% water changes daily. It has always worked for me.
Could you post a picture?

 

Yeah I'll post a picture to show when I got him to now and also how long would I do the changes for and would you recommend cleaning the filter everyday and the gravel or just purely the water?

 

Here is my betta now with the fin rot obvious on the back fins.

 

Here was the fish when I had it for about a month

 

I prefer to use a siphon and siphon out muck from the gravel at the same time but just water would probably be fine

 

Cool thank you for the help