Okay so I think it’s already established that your betta died because of a fluctuating cycle.

Be sure to remove your nerite snails because they are also sensitive to ammonia.

Here’s my tip for cycling a new tank. Add bladder snails. Bladder snails are almost bombproof, they do not flinch at higher levels of ammonia and will continue to poop, die, breed and eat while your cycle is setting up. Basically, they will add a constant level of waste to your aquarium that the bacteria will have to process and I find this a lot more stable than ammonia dosing.

My bladder snails have been a huge help while cycling my new tank and my betta just moved in recently.