Par meter readings

[Baldy]

I know im going a bit further than most, but out of curiosity i bought a par meter and it arrived today. so i built a holder for it and was playing around with it in my tank this evening.

I dont have really high lighting requirements as i mostly have soft corals. the only sps i have is a green birdsnest and a forest fire monti digi. but i was surprised how low the apogee meter read. middle of my tank which is the top of the rockwork on my 75g read 175 par. the frag shelf where im growing the sps frags are only 100. just under the water at the surface read 450. this is with my LED fixture maxed out. i usually run it a fair bit lower than that (royal blue at 70% neutral white at 90% currently, but higher than that up until a couple weeks ago).

Lately i hadnt had good coralline growth and i noticed that when i lowered my light level to acclimate some frags, when i reached my current level (RB 70%, NW 90%), the coralline growth took off, so i left it where it is. the par measured a bit lower than above readings yet! how are there people that get 500+ par and still get coralline growth?