What's wrong with my betta fish?

I got a betta fish last night and put it in one of the betta set ups they sold at the pet store. I can't remember if he looked like this yesterday, but today he looks very red around the gills. I have a test kit that checks ph, water hardness, nitrate levels, nitrite levels, and alkalinity. I tested the water and everything looks fine, but the water is hard. I read online that the redness might be from ammonia, so I'm gonna get a test kit for that. It's been less than 24 hours, does ammonia cause symptoms that quickly? I'm also going to buy some betta water, because it's probably better than mine, although mine is treated. Are there any other things that could be causing the redness around his gills? I'm really worried, so please help me out!