The small hole appeared overnight. I was testing the water ammonia level last night and it showed .50 (which I think is wrong because I was suspicious and tested my tap water and it showed the exact result... another story though). Regardless, I did a 50% water change.
Before the change, he did seem a little spastic, but he had made his first bubble nest so I thought he was alright. Then I woke up this morning to find he developed a small hole on his top fin. How could this be? It doesn't look like a tear... I'm quite sad and worried.