help please

so this is my first time growing and ive run into some problems. my plant was doing fine until the bottom leaves started to like rot or something. and it would be awesome if someone would help me to keep it alive.

it could be nute burn what you usin an how much?

peace

1 tsp of ammonium solfate was added yesterday. but it started happening like 3 days ago

Looks like phosphorus deficiency. Flush, then feed a balanced fertilizer. Should clear up pretty quick.
Fred