help with betta fin rot and the nitrogen cycle

[coltin]

Honestly, I'm having trouble concentrating on all the advice, my brain is a bit frazzled. But looking at the picture of your betta, it seems like there are some bites on its tail. Since you mentioned it's the only fish in the tank, I'm guessing it might be nipping at its own tail.

For the water parameters, you want to aim for ammonia and nitrite to be at 0, and nitrate to be below 20ppm at all times. If your readings are above this, it's best to do a significant water change to bring the levels down. Also, make sure to dechlorinate any new water before adding it to the tank.