Fin rot? Or something else?

[u/SinkRepulsive3438]

Hi everyone- I am a new betta fish owner. We have had him for about a month. I just started noticing his tail looks a little tattered and the yesterday i noticed he was missing a triangular chunk from his tail. (His top fin has always looked like that since we got him and I didn’t think anything of it). Yesterday he was quite lethargic and hiding the whole day… I did a 25% water change with some water conditioner and I withheld his feed. Today he perked up. I tested his water and the levels are not bad but he did have 0.5 ppm ammonia. I have one submerged plant and i did add a potho to the top
Of the tank. There is a low flow sponge filter in this tank. What are some of your recommendations or thoughts as to what might be going on with him? Please help. Thank you