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At the club meeting last night, I did a presentation on measuring PAR in our reef tanks and why it has value, regardless of the type of lighting choices we employ. I do try to focus on daylight spectrum more than 20,000 Kelvin, because PAR meters are designed to measure sunlight and not the blue spectrum.

Here's the PAR measurement of the frag tank. The light is a Reefkoi Evolution LED fixture that is three years old. The numbers are:
0" - measurement at the surface of the water.
1" - just beneath the surface.
6" - at the substrate.

I measured multiple spots in the tank and it varied between 120 and 170 PAR. The corals are doing quite well under this fixture, which has both blue and white LEDs. This fixture runs blue lighting from 2:30 to 9:15 p.m., and the whites from 2:30 to 8:30 p.m. The blues-only period for the final 45 minutes is quite nice to see.

The 60g cube measurements are next. This tank is lit with an Ecotech Radion Gen2 fixture. I'm running the pre-programmed Radiant Color schedule, offered within EcoSmart Live. The light comes on at 1 p.m. and gradually ramps up to create high noon from 4 p.m. to 7 p.m. and then tapers off to darkness around 10 p.m. I really enjoy the deep blue Full Moon lunar phase that I see for two successive nights every few weeks, which coincide with the full moon outdoors.

Measuring the PAR around the substrate, it averages 100-120. The clams are doing doing well, as is the large acan in the front right corner. The midpoint of the tank measured over 200 PAR, and the tentacle tips of the upper Rose anemone were enjoying PAR readings around 360 as they stretch up daily.

Remember, 0" is at the surface; 1" is just beneath the surface, and then measurements are taken every 6" until the substrate.

The 400g was measured under 10,000K lighting and then again under 20,000K lighting. I'm running two 250w TwinArc metal halides and a 400w TwinArc in the center. TwinArc bulbs switch color simply by turning them off and on during the middle of the lighting period. I really enjoy seeing the tank in both phases each day. Here's my previous blog about these particular bulbs: http://www.reefaddicts.com/entry.php...l-halide-bulbs

These are the readings during the 20,000K lighting period. Notice that the left bulb's PAR is greater than the right bulb's. I may try swapping those two ballasts to provide more intensity on the LPS bommie area; my guess is that this is ballast-related although they are the same age and brand/model.

Measurements in the middle of the tank are always stronger because of the 400w bulb both in the 20,000K spectrum and the 10,000K spectrum.

In the past, I always ran a (blue) 20,000K bulb which needed the extra wattage to level out or equalize the PAR between the two white outer bulbs. That combination worked out nicely, but with the TwinArc bulbs my tank is always brightest in the center since the colors are all the same now. Currently I'm trying to decide if I want to increase the outer bulbs to 400w for increased growth, or lower the middle bulb to 250w.

Measuring again under 10,000K lighting:

I measured a few choice corals to see what type of intensity they were receiving, and those with great growth are in high PAR spots. I also measured various areas around the substrate out of curiosity. There are chalices on the sand, including one in the back right corner that receives about 90 PAR yet still growing nicely and has good coloration. Taking readings in a living reef isn't always easy, as the rockwork and corals create obstacles... the sensor can't be placed in a fixed spot and then raised and lowered in a straight line. Working my way through the reef, I make sure the sensor is pointed toward the light source as best I can.

The big staghorn coral in the center of the reef is exploding with growth on the left side (which is directly under the 400w bulb) while the right side of the same coral is situated between the two pendants, thus in a lesser lit area. The left side measured 630 PAR, while the right side measured 300 PAR. Lots of new growth is happening in the 600+ PAR swath of light, including two Montipora species and even a cluster of candycane coral placed in the same area. Thus, I believe this coral loves intense light.

The Green Slimer frag I got a couple of months ago is bursting with growth. It's in a great spot (middle point of the tank) at 360 PAR. With measurements like these, it seems to be more logical to increase the rest of the reef to 400w bulbs instead of decreasing the wattage in the center section when growth is clearly present with no negative response.

These bulbs are in Lumenbright reflectors that place the bulbs 20" off the water. Heat has never been an issue for my tank, which during the summer averages between 79°F and 81°F so I don't think increasing two bulbs by an additional combined 300w of light will cause an area of concern. These MH bulbs were installed Aug 15, 2013 so they are just about to reach six months of use, running six hours a day.

No decision has to be made immediately - I'm just thinking about it.

Bottom line, corals / clams / anemones / LPS seem to do quite well in my tanks with PAR ranging between 100 - 130, while SPS seem to really thrive in zones that range between 300 - 600+ PAR.