Yep I think you've got some fin rot going on there. Please fill this out so we can assist:
How big is the tank?
Is it uncycled or cycled?
How much water do you change?
How often do you change the water?
Please try to answer these to the best of your ability. If you do not have a test kit, please bring a sample of tankwater to the nearest local pet store. They should test it for cheap, but most of the time free. Write the actual numbers down, don’t take ‘okay’, ‘low’ or ‘fine’ for an answer.