Missing neon tetra?

I bought 4 neon tetras last night, and i woke up this morning and there was only three? I cant figure it out, would my betta of ate it? or would my corydoras of ate it if it was on the bottom dead or something? I know my otocinclus couldnt of done it. I took apart my power filter and there was no neon tetra in the filter, I looked all over the gravel its driving me crazy, what can it be?!?! plz help if you have any idea:S