The light is 15cm from water surface.
- 1cm above water: ~200 umol
- water surface: ~151 umol (sensor fully submerge)
- 20cm below water: ~46 umol
I surprise there is about 50umol drop between above and under water although it is roughly about 2cm different in distance from the light.
You can also do something like below which my friend did on his 3ft tank