This is what a quick google search came up with for ingredients...
3 cups unbleached flour (Not good for water quality)
2 tsp. baking powder (No idea... Probably not good though)
1 tsp. salt (Too much salt isn't good for a betta's digestion)
4-6 Tbsp. vegetable shortening or lard (Um... that can't be good...)
about 1 1/4 cups warm water
If the ingredients didn't kill him, the debris that leeched into the water did. Tortilla's break apart pretty easily and it would be almost impossible not to have some remnants of it in the water.