Home Mix Room Small - 100hz null advice

To find the source of a null, you first must find the wavelength of the frequency. You have an issue at 100Hz. The wavelength is 11.25'. Divide by 4 to get ~2.8' or 0.86m. It's likely that some reflective surface 2.8' from your monitors or microphone is causing this destructive interference. I assume your measurement is overlaid Left and Right so this boundary is equidistant to both monitors. The best thing is to move your monitors, the second best is to absorb the boundary. For 100Hz, you need 6" rigid, 12" safe n sound or 24" fluffy, each ideally gapped 1x thickness but the second and third options might not need it. 

You also have a touch of ringing around 120Hz. I've posted how to find your room's modal frequencies elsewhere on this forum. You can search for the formula if you need it.


calculators.io/wavelength-to-frequency/

Also, what's the time on that waterfall?