Is this fin rot?

Hello, I've been struggling with my sick betta fish few weeks ago. His health progressed after clean water change and new filter.
However, he is not fully recovered, and I noticed black part under his belly (where the pelvic fin is attached)
Is this fin rot?
I carefully check his past pictures, and he had some black since the day he came to my house. I think it just got darker and bigger? And one of his pectoral fin was half ripped off before he came.
Several days ago, I went to LFS and bought some anacharis for him to rest. He liked the anacharis, but few days after, the tank started to smell like that LFS. Like some soil fishy smell. So I removed the anacharis and did 30% water change, but the smell is still there.
Some of the rocks that were white changed the color to blackish white. I don't know if it is normal.
Is this fin rot? I would like to make sure before any treatment.
I had him for 1 and half month.
Currently there is sponge filter, preset heater (78 degree), 5 gallon tank (water is about the half)
I'm very cautious to do water change because he seems very stressed out whenever I do water change.
Please let me know if this is fin rot or something else. Thank you so much for your help.

His pelvic fin was always curled, it was even smaller when I first got him, so I don't think that is not the issue. What worries me is the black color between the pelvic fin.

Fin rot is caused by poor water quality and a dirty tank that encourages bacteria to grow and infect the fish. The easiest way to prevent this is with big regular water changes, gravel cleaning and an established biological filter.

The fish in the picture does not appear to have fin rot.

If ornaments, rocks, etc in the tank are going black, then there is something wrong. If things are rotting in the gravel or on rocks, it will cause bacteria to grow. If the oxygen levels are low, the area can become anaerobic and turns black and smells bad.

If you have any black rocks or ornaments, take them out and hose them off. Reduce feeding and do big (75%) water changes and gravel clean the tank a couple of times a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

If the tap water has chemicals in, it might cause the fish to stress out after a water change. You can try holding the tap water in a container for a few days. Add a dechlorinator and put a filter with carbon into the container. Aerate it vigorously for a couple of days and then use that water for water changes. See if the fish is ok with that water.

Colin_T said:

Fin rot is caused by poor water quality and a dirty tank that encourages bacteria to grow and infect the fish. The easiest way to prevent this is with big regular water changes, gravel cleaning and an established biological filter.

The fish in the picture does not appear to have fin rot.

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Hello Colin thank you for your reply.
What would be the blackish color in his belly? the part where its circled in blue.
His past pictures are taken from the side so I can't compare the belly but I feel its getting more obvious to see.

There are just some rocks with color change. I took a picture and threw them away. They were mainly single color I remember.

I'm following all the rules for the water change.
Putting a light pressure air stone would help? I have air stone that came along with the airpump.

no idea about the black stuff

the stuff on the rocks looks like algae

an airstone will help circulate the water and increase the oxygen levels in the water, but don't have too much water movement otherwise the fish will struggle to move about.

I agree, your betta does not appear to have fin rot.

Colin_T said:

no idea about the black stuff

the stuff on the rocks looks like algae

an airstone will help circulate the water and increase the oxygen levels in the water, but don't have too much water movement otherwise the fish will struggle to move about.

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Oh ok thank you, I have valve so I will minimize the air pressure.

PheonixKingZ said:

I agree, your betta does not appear to have fin rot.

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Thank you to let me know, I appreciate all the help from this forum, thanks!

You are welcome!