Damage to angelfish fins isn't uncommon at all. They can rip one just "playing around" the tank, or in moving them (net damage) and so on. Thing is - fin rot resulting from that damage is nearly always connected with water quality issues. SOMETHING not right, and damaged fin becomes a bigger issue. #1 priority - check and recheck water parameters, and increase the pwc schedule.
I have dealt with damaged fins and fungus/rot and have only treated with meds once - and that was with a high-dose "dip/bath" with salt and Tetra Fungus Guard. Symptoms disappeared a day later with the fin now completely healed. Since then, have had a couple of slightly torn fins that have healed without any sign of disease (and water quality has been much more stable and good in that time).