You still need to have your water tested for ammonia, nitrites and nitrates as a matter of urgency. Fin rot generally results from dirty water so you need to see exactly what the quality of your water is. If you don't have a liquid test kit, take some water to your local fish shop for testing. Make sure they give you the actual measurements rather than a 'that's fine' and don't get sucked into buying products to correct 'this and that'.
If you have readings for ammonia and nitrites then there is a cycling issue in your tank that needs to be addressed, and it will be the most likely cause of your fin rot.
