possibly diseased betta

[xerxeswasachump]

So i checked out the disease link in the sticky and i can't really identify what is wrong with one of my bettas.
So i have 4 female bettas in my comunity paludarium. The dominant one has suddenly become much less social than usual and her fins look frayed in a line. This is definitely not due to combat since she is the dominant one and nipped fins from fighting usually are in single spots, not in an even line along a fin.
She has also started sticking closer to the surface and seems to maybe have some trouble keeping her abdomen from rising. There is no swelling or outward appearance of illness except the line of possibly rotting in her abdominal fin.

My pH is around 7.4 and my water is very hard. There are 0 nitrites and about 80ppm of nitrates. I know that is high but i just did a water change and i have no explanation for it, except that i just took out the carbon from the filter on the recommendation of some people. My tank is also heavily planted so i figure they can take care of it. I would rather not put the carbon back in the tank because accessing the filter causes quite a disturbance and i hear it wears off in a week anyway.
Would using medication destroy my cycled filter?

[Sprite42]

If it is just the betta that is ill, you should quarantine her in a separate tank. There is no need to treat the whole tank if she is the only one having problems. How old is she?

You may also need to do some more water changes to get those nitrates down.

[xerxeswasachump]

I will do that if i can. But anyway, I'm gonna try the pea method first just to see if it is a digestive problem.