2 quick questions about PAR testing
 

Have some time to figure out height that I will need for my lights, but have 2 basic PAR questions

1) I should set my lights at highest setting I will use during the day (ex: 50% blue, 50% white), and take PAR readings. Correct?
2) Does it make any difference if salt vs fresh water is used? Reason I ask is because tank is on the floor in the garage, and I will move it many more times before it is ready for rocks, sand and saltwater. I dont want to mix saltwater that I will dump after PAR reading are done (dont have anywhere to store it). Waste of 100G of salt and RODI water.

I know I could just wait till tank is setup, but since I have time I wanted to use it to dial in lights as best as possible.

Thanks

You dont do par readings until you have your rock set. Height of the rocks (even a couple inches) can be the difference of 50 or 100 par.

Dont do it until the tank is aquascape and running.

Thats a valid point. My thought process was that if I did initial test with empty tank, it would at least give me good starting point. I know that once rocks are added that will change PAR reading around them, but PAR should remain the same above rockwork, no? Tallest rock structure will be around 14" with sand bed, so from top of the rock work to top of the tank (24") should have same reading regardless of rocks. Am I missing something?

Thanks

PAR output is affected by all sorts of things. Adding carbon will increase your par since the water will be more clear. PAR will drop over the course of a few weeks after installing new T5HO bulbs. Even changes in circulation, the turbulance on the surface, affects PAR. The type of lighting makes a difference, too. Point source LEDs like a kessil mean you can get 250 PAR when the sensor is facing towards the light and 80 PAR when it's facing 90 degrees from the light.

Also, PAR isn't the be-all-end-all. Chlorphyll photosynthesizes best at certain wave lengths: https://www.khanacademy.org/science/...hetic-pigments

A very blue light at 200 PAR might be blasting your corals compared to a white light at 200 PAR.

There are also species (typically deep water) which don't like red spectrum light. Here's one study corellating red light with poor coral health: https://pubmed.ncbi.nlm.nih.gov/24658108/

Deep water acros were best under blue spectrum, grew at half the speed with the same par but half blue and half red, and died under red. Shallow water acros grew at the same speed with either blue or blue/red, but also died under just red.