PAR vs. watts per gallon?

[Michael Hughes]

As a new reefer, I'm still trying to figure out lighting. I have had very different results with corals between my frag tank and DT, despite the PAR being in the same range. I'm using the same salt in both tanks (Red Sea blue bucket).

125 gallon DT

• Has been running for 8 months
• Two Nicrew HyperReef 100W gen 2s
• PAR is 100 at the sandbed, 110-140 on top of the rocks, and 250 just below the surface

14 gallon (cube) frag tank

• Has been running for 14 months
• One Nicrew NavaReef 65W
• PAR is 60-100 on the sandbed, 120-150ish on the rocks, and 180 just below the surface.

Virtually anything I throw in the frag tank thrives. LPS, softies, zoas, SPS (Pavona, Stylophora, Anacropora, Montipora), encrusters, gorgonians, etc. This is despite only running a tiny HOB, doing small water changes once a month, and running the tank fishless. I also have had my alk dropping due to all the growth and plan to start kalkwasser soon to replace alk. The only coral I've lost in the frag tank was an acro (the first I've tried), which unfortunately took it hard when the salinity rose too high after my refractometer drifted off calibration.

On the other hand, corals I transfer over to the DT are a mixed bag of results. Leptastrea has been bulletproof, Leptoseris, Oculina, acans, and favias are looking fine, but grow at a snail's pace compared to the frag tank.

Anacropora, Stylophora, bird's nest, montis, and scroll coral all seem to do fine for the first month or so and then decline before slowly losing tissue and dying.

These results suggest to me the lighting is too low in the DT - it seems the corals that need more light are starving, and the ones that are fine with less are surviving but not really growing much.

So this brings me to my question. The PAR is so close between these two tanks, and the lights are same brand (and run on the same schedule), yet the major lighting difference is watts per gallon. The DT is 1.6 watts per gallon and the frag tank is nearly triple that at 4.6 watts per gallon... yet the PAR readings are virtually the same.

Is watts per gallon a more meaningful metric than PAR? If so, should I add more lighting to the DT?

 

[BeanAnimal]

Gallons doesn't tell you area. More appropriate would be watts per square foot or similar for surface area of the tank, but that too is misleading because of light fall off as a product of horizontal distance from the lamp luminaire.

If both lamps are set to the same spectrum and average spectral power readings are similar, then the difference is likely water parameters, biology and flow.

 

[Michael Hughes]

Gallons doesn't tell you area. More appropriate would be watts per square foot or similar for surface area of the tank, but that too is misleading because of light fall off as a product of horizontal distance from the lamp luminaire.

If both lamps are set to the same spectrum and average spectral power readings are similar, then the difference is likely water parameters, biology and flow.

Thanks for your feedback. The frag tank does have a 6 month head start, so I've been keeping that in mind in terms of the biology.

For what it's worth, the DT is getting 22 watts per sqft and the frag tank is getting around 60, but I suspect there is even more light falloff in the DT due to its length. So by this metric the frag tank is receiving 3+ times more in terms of wattage.

 

[BeanAnimal]

Thanks for your feedback. The frag tank does have a 6 month head start, so I've been keeping that in mind in terms of the biology.

For what it's worth, the DT is getting 22 watts per sqft and the frag tank is getting around 60, but I suspect there is even more light falloff in the DT due to its length. So by this metric the frag tank is receiving 3+ times more in terms of wattage.

Right but if the spectrum is the same, then the light meter (PAR, Lux, whatever) is telling you the real comparative light energy at that spot. If they are the same, they are the same. Depth and angle can shift spectrum a bit, but not enough to matter at the depths and angles we're are talking about.