My baby Pooh died last night

I got sick Tuesday and wasn't able to get the water test like I had wanted to. By the time I tested the water last night it was .50 for ammonia. It's a 5 gallon tank. That must have been what did it.
However, she was clamping up before I gave the tank a 100% water change on Monday night.

I did use water conditioner but I don't have any other additives right now.
Anyway I will be getting a new betta but not until I get the tank right. Poor Pooh was a victim of my ignorance.
Pookie the cory catfish seems fine, are they affected by ammonia?