In the first photo I see the black at the tip of his tail which looks like fin rot, but in the flaring shot I only see maybe a bit of tail biting.
Can you please fill out this form:
How many gallons is your tank?
Does it have a filter?
Does it have a heater
What temperature is your tank?
Does your tank have an air stone
or other type of aeration?
Does your Betta have tank mates? What kind?
brand do you use?
Do you feed flakes or pellets?
How often do you feed your Betta? How much?
your Betta became ill how often did you perform a water change?
What percentage of water did you change?
Do you vacuum
the substrate or just dip out water?
What additives do you use?
What are your water parameters? Please give exact numbers. If tested by pet store
please get exact numbers. "Fine" or "Safe" won't help us help you.
Symptoms and Treatment:
When did you first notice the symptoms?
How has your Betta’s appearance changed?
How has your Betta’s behavior changed?
Is your Betta still eating
Have you started treating your Betta? If so, how?
Does your Betta have any history of being ill?
How long have you owned your Betta? Was he or she a rescue?