Noob question - PAR ratings with more than one light present

[WiscoFishNut]

Be gentle lol..

Probably a dumb question, but I didn't want to make any incorrect assumptions

For my coral QT tank (20g long) I plan on running two Fluval Sea Marine 3.0 LED fixtures to provide lighting. Fluval claims that the lights measure 90 PAR @ 12". The lights will sit directly on the tank, which is 12 3/4" deep. Question - Would corals sitting on egg crate on the bottom of the tank get around 180 PAR, or does the math not work that way?

 

[Huff747]

Math doesn't work that way. I'd be shocked if you get exactly the 90 par @ 12" because everything about the tank impacts it. Surface ripples, whether you have a screen top, if your water is more/less clear than their water, how clean your glass is, what settings you run the lights at, etc.

Even if you did get exactly 90 at 12 inches that's probably only directly under the fixture, it'll go down as you spread out from under the fixture. There will be places where the light from the two fixtures overlap that will probably be higher than one fixture's max but how high that goes will depend on how close together they are and all the other factors.

 

[Hemmdog]

I’d be surprised if you got over 60 par.

 

[WiscoFishNut]

Math doesn't work that way. I'd be shocked if you get exactly the 90 par @ 12" because everything about the tank impacts it. Surface ripples, whether you have a screen top, if your water is more/less clear than their water, how clean your glass is, what settings you run the lights at, etc.

Even if you did get exactly 90 at 12 inches that's probably only directly under the fixture, it'll go down as you spread out from under the fixture. There will be places where the light from the two fixtures overlap that will probably be higher than one fixture's max but how high that goes will depend on how close together they are and all the other factors.

I’d be surprised if you got over 60 par.

Perfect, that's what I needed to hear! So for those lights to be even remotely viable I need to raise the frag rack up to a minimum of 6 inches I'd guess, maybe more? I'm only looking to start with softies and LPS. The manufacturer states that @ 3" PAR = 376. Do I need to be worried about heat or anything putting the rack that far up in the water column(6")?

 

[Hemmdog]

Perfect, that's what I needed to hear! So for those lights to be even remotely viable I need to raise the frag rack up to a minimum of 6 inches I'd guess, maybe more? I'm only looking to start with softies and LPS. The manufacturer states that @ 3" PAR = 376. Do I need to be worried about heat or anything putting the rack that far up in the water column(6")?

Nah, not with leds.