Betta Fish: Anyone have a clue what could have happened?

I’ve had my male beta since December his tank crashed in early January. After 2 months his tank fully cycled and for the last 2 weeks everything was perfect and he was happy and healthy, eating properly, very active etc. Water readings were from what others said perfect. Ammonia 0 nitrites 0 nitrates 5-20ppm PH 7.4-7.6

I did a 30% water change last night at 11pm when nitrates spiked to 40ppm. I buffer my water to KH 2 as my tap water does not have KH and it does have 0.25 ammonia so I prime and buffer new tank water every single time. Ammonia always goes down to 0 after a couple hours.

Today at 3pm I went to turn his lights on and I noticed he rapidly swam to the surface and was gasping for air like crazy then immediately sank I observed for a few minutes and noticed he was tilting sideways then swimming rapidly then dropping face down again. At one point he was pointing directly down in his plants. I scooped him up and brought him tot he surface for air and immediately prepped a jar to put him in because I couldn’t see swim bladders signs so I knew something must have happened in the tank water. I put about 2 inches of water in the jar and added a tiny bit of pine t. He was still rapidly swimming but less then what he was. He was trying to sit upright and then would just sink to the bottom. I did an emergency 40% water change incase it was toxins in the tank that my readings couldn’t pick up. Long story short he passed away. What possibly could have caused this? I’m heartbroken and feel like I’ve failed. I did everything I was told to do I followed every single thing that was recommended to give him a great life. 10 gallon planted tank. I’ll post some photos of this final moments . I’ll also add one for before all of this started when he was at his happiest

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