... I noticed the other day Sushi, my Betta ...
Hi and welcome to the forum
Did you really name your fish Sushi?
Fin rot and most other fish health issues are caused by poor water quality. Having a good filter on the tank and doing regular water changes will normally keep the water clean and the fish healthy. It normally takes about 4-5 weeks for the filter to develop enough good bacteria to keep the water clean. During this time the ammonia and nitrite levels can go up and seriously damage or harm the fish.
Do a 75% water change and gravel clean the substrate each day for a couple of weeks or until this is resolved.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
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How long has the tank been set up for?
How long have you had the fish for?
Is there a filter on the tank and is it run continuously (24/7)?
Have you tested the water for ammonia, nitrite, nitrate & pH?
If yes, what were the results in numbers?
If you don't have test kits, you can take a glass full of tank water to the local pet shop and ask them to test it for you. Some pet shops charge a small fee to cover the cost of their test kits. If you do get the shop to test the water, write down the results in numbers, when they do the tests.