[Ninong]

Originally posted by CaptainK

I was wondering if anyone knew which had a greater PAR rating:
Power Compacts or VHO??

I don't remember seeing these figures, just lumens for PC vs. VHO. Anyway, it's probably pretty close and the difference, if any, not worth considering. There might be a noticeable difference in spectral distribution between super actinic VHO lamps and some PC actinics. Might be. Don't know if that is something to be concerned about or not. Probably not.

Is there any general rule of thumb as too the amount of PAR per gallon or anything like that?

No, thank God! And if there were, it would be as useless as the watts-per-gallon rule of thumb! Just think of it this way: all corals have different light requirements. The most light hungry corals, Acropora spp., reach their saturation point in a range of 20-30% PARS. Definitions: saturation point is the maximum amount of PAR that an animal can use, anything above that does not increase photosynthesis and, in fact, has to be dealt with by the coral. Too much light and you reach the inhibition point, which is where the coral starts shutting down (or even bleaching) due to photo-inhibition. PARS = photosynthetically active radiation at the surface.

Can you provide too much light over an aquarium? If your definition of "too much light" means more than maximum tropical sunlight at the surface at midday at the equator, the answer is "not with 400w 6500K Iwasaki, or equivalent, lamps." But these would come close. You could exceed tropical sunlight with 1000w or 2000w metal halides if they were located too close to the surface of the tank.

Do you really need all that light? No, of course not! You can get by with a LOT less. A lot!

Because corals do not require saturation point lighting to thrive. There is a much lower number called the "compensation point," that is the starting point. Corals require more light than the compensation point in order to grow and reproduce. They will show increased growth and health as the light levels rise above this compensation point because the additional light results in increased photosynthesis... up to the saturation point, where additional light does not result in increased photosynthesis. Below the compensation point, corals die. Besides, corals will photo-adapt to the available light anyway, within reason. You want to be somewhere in the middle between the compensation point and the saturation point. If you want crotch-rocket lighting, you might want to go for the upper end of the range of the most light hungry corals: Acropora spp.

So, since most Acropora spp. reach saturation at 20-30% PARS, all you really need (maximum) is about 20% PARS reaching about 12" into your tank. You may find it easier to deal with lumens instead of PAR in trying to calculate this because most scientific papers record light measurements in watts/m2 instead of the more familiar PPFD (photosynthetic photon flux density) and most hobby books seem to report light readings in lux instead of PAR.

Remember that maximum light on the reef is only during the three hour period from 11 a.m. to 2 p.m. -- it builds up to that level rather quickly and then starts dropping off slowly after 2 p.m. So, if you see a figure in a book that says somebody measured 120,000 lux at the surface at noon on a clear sunny day, just remember that the reading 1 meter below the surface will be about half of that figure, and also remember that the average reading, due to clouds passing overhead and stormy days, will be at least 1/3 lower.

Here is a measurement from TRA-1, p. 179: In the northern Red Sea, 120,000 lux at the surface, 50,000-70,000 at the reef crest, 15,000 lux at 15 m (50 ft.), and 2,000-4,000 lux at 20 m (65 ft.).

Daniel Knop has used 250w, 400w, 1000w and 2000w 10,000K Osram metal halide lamps over his tanks and according to him, a 250w lamp produces 19,000 lumens, a 400w lamp 33,000 lumens, a 1000w lamp 80,000 lumens, and a 2000w lamp puts out 170,000 lumens. Then you have to allow for the loss of intensity cause by attenuation in the water. Lets take the figure of 50,000-70,000 lux at the reef crest and multiply that by 20% = 10,000-14,000 lux to provide saturation point lighting for most Acropora--this would be considered a crotch-rocket lighting system! Or, you could take the figure of 15,000 lux at 15 m (50 ft.), which would take in most corals collected for the hobby, and multiply that by 20% = 3,000 lux. So you need something in between those two extremes (3,000-14,000 lux) reaching the coral. All depends on your budget and your ego.

It is hard to find lux readings online for metal halide lamps at various depths in aquariums, although I have seen such figures for fluorescent lamps. You can maintain almost anything with PC fluorescent or VHO fluorescent lighting, although Tridacna clams and certain SPS corals would do better under metal halides. Naturally, you would have to be selective in where you placed SPS and clams if you are going with lower end lighting. Daniel Knop says that you need a minimum of 250w metal halide lighting for Tridacna clams to really thrive. I'm sure he knows what he's talking about... especially when it comes to T. crocea and T. maxima that are found high up on the reef.

If you browse the various European and Asian reefkeeping bulletin boards, you will see dozens of very nice reef-tanks that are lit with much, much less light than we would consider necessary over here. In fact, just look in MCRA-3 at the section on hobbyists' tanks and you will see lots of 5-ft long, 150-gal tanks that are lit with only 2*150w HQI lamps or 2*250w HQI lamps, or even some that are lit with only fluorescent lamps. Granted they aren't as colorful as we are inclined to want and they contain mostly softies, but they are doing well for several years nonetheless. You will also find a few European tanks online with crotch-rocket lighting (1000w metal halides).

Ninong