Betta Fish: HELP! Finrot and crashed cycle

this is pebbles, he lives in a 14 gallon tank which was once beautifully cycled never left 0ppm no matter what however one day i decided to switch his filter media and i learned quickly that i should never have done that as it crashed his cycle and it has persistently caused ammonia buildup despite daily 50-75% water changes, unfortunately this caused pebbles some finrot which i am trying to aid with extra tannins and trying to manage the ammonia but the issue is i ran out of water conditioner unexpectedly today and tommorow is easter so every store that would carry it near me is closed, i ordered it on Amazon and it will be here Monday but i am scared that the ammonia will be bad again tommorow and i wont be able to change it, does anyone know what to do in this situation?? its a heavily heavily planted tank but they dont seem to be helping much in this case