Lighting for my 37 Gallon.

[skyhawk8519]

I just purchased a 150watt MH for my 37 gallon.
It is a 20x18x24 deep tank. How much light am i going to loose by the bottom of the tank. Anyone have a 24" deep tank with a 150?

[pjf]

Here’s a rough cut. I’m sure that there are better analyses out there. If anyone wants to improve the calculation, please chime in and show your math.

1. Let’s assume that your 150-watt lamp is a typical shielded 10K DE lamp. Per Dr. Joshi (http://www.advancedaquarist.com/issu...2/feature2.htm), it produces a PAR of ~60 µmol•m2•sec at an 18� distance away.
2. With a perfect parabolic reflector above your lamp, your aquarium will now receive 120 PAR at an 18� distance away from the lamp.
3. Your fixture is 6� above the water so the reference point that is 18� away from the lamp is 1 foot under water. Per Dr. Joshi (http://www.advancedaquarist.com/2005/8/aafeature), 2 feet of seawater absorbs 14% of the PAR, so 1 foot of seawater will transmit 93% of the light for a PAR of 112.
4. Half of that light (56 PAR) came from the reflector and is parallel (like laser light), so another foot of seawater transmits 93% of that light to the bottom for a PAR of 52.
5. Half of the light (56 PAR) came directly from the bulb and is subject to the inverse square law. But since your tank’s glass walls result in total internal reflection (TIR), none of it is lost. (Sorry, but you can do the math yourself. Don’t forget that the surface will bend the light closer to vertical so it is almost parallel anyway.) Another foot of seawater transmits 93% of that light to the bottom for a PAR of 52.
6. Adding up the reflected and directly received light, we have a PAR of 104 at the bottom of your 24� deep tank.
7. With a PAR greater than 100 µmol•m2•sec, your SPS corals are receiving too much light and are starting to exhibit photo-protection (http://www.advancedaquarist.com/issu...04/feature.htm).
8. You add live rocks to shade light reflecting off the glass walls of the tank to protect your SPS corals at the bottom. This reduces your PAR to a safe level well below 100 µmol•m2•sec.
9. SPS corals at the top of the tank are still being photo-inhibited so you reduce your MH photoperiod and add actinic tube lighting. (In nature, SPS corals are photo-inhibited by mid-day sun and depend on morning & afternoon light for photosynthesis.)