When you measure PAR in planted tanks full of water there is almost no way to standardize the readings. Some tanks have lots of plants shading parts of the tanks, others don't. Some tanks have very clean glass, inside and outside, and others don't. Some tanks have tannin stained water and others don't, etc. That's why I only use data taken in air. It is then fairly easy to correct those PAR readings for various sets of tank conditions, and you then have PAR numbers that are specific for the light only.
If you get PAR readings for 3 distances from the light, say 6 inches, 12 inches and 24 inches, it is easy to determine the PAR at any other distance. A reading at each end of the light and at a few distances off the center of the light, front to back, will complete the characterization of that light.
One thing I just discovered, accidentally, is that you need to measure PAR in an area with no white walls nearby. I used a hallway for my last measurements, because it was dark there, but the white walls distorted the readings by reflecting a lot of light.
Where ever you take the measurements it is best to start with measurements with the light turned off, then subtract that PAR from the readings with the light on. Generally that will be about 1-5 mms of PAR to be subtracted - important for low readings and irrelevant for high readings.
With LEDs, you need to include the LED current with the readings, since PAR varies with current. And, to be really useful, you need to include the LED spacings in the readings.
Of course, all of this is relevant only if you are seeking data about the light. If you are seeking data about the tank and the plants in it, you just measure PAR in the filled tank at various locations of interest.