Sadly, cleaning the tank in that manner will likely make the problem worse because the cycle was basically reset.
Fin rot is caused by poor water parameters which is why I asked about the ammonia, nitrite, nitrate, and pH readings. With the cycle reset it will be more important now to test the tank regularly and change water when ammonia or nitrite rise above 0.25ppm or nitrates rise above 40ppm.
What are you using to treat the fin rot?