FFT of a square wave
I'd make this a comment, but I don't have enough points to do that yet.

You should plot your FFT data starting at 0 Hz and go up to, say, 500 Hz.  That will give you 10 or so harmonics.  You are probably zoomed too far out in the frequency domain(x-axis) to get much detail and realize what's going on here.  

And yes, the fundamental should be at 25 Hz.