My betta unexpectedly died last night.

I have absolutely no idea what went wrong. I've only had him for about 3 months.

10 gallon

Cycled

pH: 7.0

Ammonia: 0

Nitrite: 0

Nitrate: 0

(Tested before and after a water change, which was conducted after I noticed my betta going downhill.)

Temp: 80F

Filtered with two sponge filters.

Contains both artificial and live plants. (Was working on transitioning to a fully planted tank)

Tank mates: 2 Apple/Mystery Snails, 2 African Dwarf Frogs, 2 Corydora Catfish (planned to get 2 more when my lfs had some in stock. I got the only two available at the time.)

All creatures were quarantined for a week before adding to the tank. Last creature was added to the tank roughly 2 months ago.

Change 25% water every 3-4 days.

No new decor/plants/anything added recently.

Yesterday morning my fish was perfectly fine. He chased my finger around the tank, jumped for his breakfast, schooled with his cory friends, and hid peas from the snails.

Last night as I was going to give him dinner, I noticed he was just laying on the gravel on his side and it looked like he was hyperventilating. I prepared the cup he lived in at the store with some new (conditioned, tested and same temp) water and transferred him to that then floated it in the tank to maintain the temperature.

He would sometimes float his head towards the surface then fall back down. There was nothing visibly wrong with him except for slightly clamped fins. His scales were fine, fins not torn or burned, no pineconing, and his gills were not discolored.

I went to bed, and when I woke up he had passed.

I'm really frustrated because I just don't know what happened.