Saltwater Aquariums 101: Lighting


Active Member
This topic is a very widely debated subject. There are many opinions, a few facts, and very little absolutes. This must be kept in mind while deciding which option is best for you.
With that said, there are a few things that must be decided upon before any lighting is purchased as there are different systems available for different types of tanks. As mentioned in previous articles, what you pick depends on what type of tank you want. So lets start there . . .
Fish Only Tanks: If you are planning on setting up a tank that is only going to house fish, then your decision is pretty simple. You can basically get away with anything you want in this type of tank. Since its a fish only tank, and you don't have any photosynthetic animals, there is no need to get expensive or high powered lights. The fish don't care . . . as long as they can see, they're happy! So if this is your tank, then instead of spending a couple hundred on an expensive light set up, go with a standard twin tube fluorescent light strip and spend the money on some really nice fish!
Fish only with live rock: This is pretty much the same as above. If you want the lights, then get em' . . . if not, don't buy em'. You can always upgrade later.
Reef Tanks: This is where it gets challenging. Reef tanks are a whole new ballgame with lights because obviously, most corals are photoysnthetic light loving animals. And since we can't shine the sun in our living room, we improvise!
There are numerous options once again with reef lighting and once again, it depends on what you are trying to achieve with your tank and what types of corals you would like to keep. To get things started, lets go over the various options available, and from there we can talk about which lights are best for which kinds of corals.
Some basic terminology:
Watts: The amount of electricity used to power the bulb. It is NOT - I repeat NOT - the output of the bulb. There is a direct relationship between power consumption and output, but Watts are NOT a measure of output. They measure input. This is one of the reasons why WATTS PER GALLON IS A USELESS RULE! Do not use it! (I'll expand on this in a minute)
Lumens: A lumen is a unit of measurement of light and is equal to one foot-candle falling on one square foot of area. The more lumens, the brighter the bulb. Lumens indicate how bright an object is to human eyes, but it is not a unit of light output, its just how bright it is. The difference is subtle but important.
PAR: Photosynthetically available radiation - It indicates the total energy available to plants and corals for photosynthesis, and is thus a key parameter for biological and ecological studies. This is a most important number to pay attention to when deciding on which lights to use. Unfortunately PAR isn't listed so you need to search for someone that has a PAR meter and has published results with the bulb you're interested in. A lot has been done with the common MH bulbs but flourescents are severly lacking.
Kelvin: This is the color of the light that the bulb produces. The acceptable range for reef aquariums ranges from 6500K (yellow) to 20,000K (blue). So the lower this number, the more yellow the light, and the higher the number the more blue. Some people use pure 20K bulbs and their tanks often appear rather blue. Others use a 10K bulb which is a crisp white. This is the most important number to pay attention to when deciding on lights because if you buy a bulb outside of the spectrum useable by corals then you are buying a useless bulb. Another thing to keep in mind is that the higher the K rating, the deeper the light will penetrate in to the water. Hence why the ocean looks blue.
Ballast: The mechanism that converts electricity to a useable energy for the bulb. This is what lights the actual bulb. They are available in a variety of types including electronic and mechanical, normal, HO, VHO, HQI, etc. The ballast you choose must match the type of bulb you use.
Well, thats the basics . . . there is much more, but at least with this info you can get the general idea. Lets start talking about some specifics!

Normal Output (NO): These lights are the lowest on the totem pole. They are the standard lights that come with aquariums when you buy them. They are really inappropriate for most set ups as they are not powerful enought to sustain corals for long term periods. However, one potential application which may find NO lights appropriate is in a refugium. They can be bright enough to grow algae which is the purpose of a refugium. The only advantage of NO lights are that they are cheap, readily available in a variety of sizes, lengths, and kelvin ratings.
High Output (HO): These are the next step up from NO lights. These lights again are available in a variety of lengths, sizes, and kelvin ratings. These are capable of sustaining some corals for a long time and can be used in some reef aquariums. A relatively new light on the market are the new T5's which are HO lights and have so far established themselves as an acceptable light source. These bulbs require a special ballast to drive them.
Very High Output (VHO): Same as above only even more powerful. These lights are again capable of sustaining a variety of corals longterm. The bulbs are readily available in a number of sizes, kelvin, lengths, etc.
Power compacts (in my opinion) are probably somwhere between HO and VHO. I personally am not a fan of PC's because of their reputation of breaking easily and up until recently they did not have a true actinic bulb available in PC. However they do now apparently.
High Intensity Discharge (HID)

Metal Halide (MH): As of now, these are the cadillac of the lighting world. These are VERY bright lights which are capable of sustaining all types of coral. They come in numerous Kelvin ratings and wattages ranging from 70w up to 1000w. The most commonly used in reef aquariums are 150w, 175w, 250w, and 400w. There are two types of MH lighting, mogul base which are large bulbs that are encased in a Glass housing that screw in to a socket similar to a standard light bulb. Then there are the HQI bulbs which are also known as double ended (DE) halides. These bulbs do not have a glass housing and thus need to be shielded before being used. They snap into a special socket. The HQI bulbs are generally brighter. Using MH bulbs will give a nice shimmering effect in the water where as fluorescents will not . . . a nice benefit in my opinion!
MERCURY VAPOR - The Iwasaki DL Mercury Vapor bulb run on a Mercury Vapor Ballast is about the best bang for the buck out there in terms of PAR per penny. Some report that it grows very healthy corals very quickly but that most SPS grown under the Iwasaki are Brown and the light is fairly yellow.
Lights NOT acceptable for reef tanks:
High pressure sodium
The reasons behind this are generally lack of PAR, wrong kelvin, or just plain impracticality.
So lets briefly return to that watts per gallon thing . . .
In addition to the notion that watts measure input, not output, another thing to consider is that as a tank gets deeper, the less light is penetrating the water. For example, if you have a 20 gallon tank that is 5 inches tall (its an example, not reality), and run 100 watts of NO fluorescent lights, thats about 5 watts per gallon. But what if the tank was still a 20 gallon tank and 100 inches tall . . . do you think that its going to be just as bright on the bottom as the 5" deep tank? I hope not . . .
Another thing to consdier is that MH light penetrates the water better than fluorescent light. So 400w of MH does not equal 400w of VHO. In sum . . . the watts per gallon rule is JUNK!
So how about corals? What lights do you need to keep what corals? This is something that is FAR beyond the scope of this thread. It would take a whole book to adequately cover this topic so I will just cover the bare bone basics . . . .
Soft corals (those without a calcerous skeleton) in general require the least amount of light. You can keep a soft coral tank with using all HO lighting.
Large Polyp Stony (LPS) corals require more light than soft corals. They can be kept under HO, but VHO would be better.
Small Polyp Stony (SPS) corals require VERY bright light that is only achieved with MH lights. SPS kept under less than MH will fail to thirive if they even survive. Clams also fall into this category.
I know that there is a LOT more that needs to be covered on this subject. Please feel free to ask questions, make corrections, ask clarification, etc . . .
Like I said, this is a highly debated subject, and I am no expert . . .


Active Member
I figured you (or someone else) would :D
I tried to keep this as basic and as light as possible because this is the newbie forum . . . . in addition, I think that this is the perfect place to discuss differences in opinion.
Lets hear it!


BTW...I put another post under equip. and diy,but I have not gotten many responses..I'm getting new bulbs Jan 3rd for my light..Do I put them all in at once or a few at a time and not leave the lights on as long..btw 2X65 10,000k & 2X65 03 actinics


Staff member
Well, lets go ahead and get the different opinions in or resolved before stickying this topic. I don't see a problem with including "points of views". This is one of the most difficult topics there is in this hobby and not easy to just cover in a post. Bang, or others, if you can add some alternative info, that would be great.


Active Member
Yup . . . completely forgot to touch on PC's . . .
Power compacts (in my opinion) are probably somwhere between HO and VHO. I personally am not a fan of PC's because of their reputation of breaking easily and up until recently they did not have a tru actinic bulb available in PC. However they do now apparently.


Staff member
Do you want to add PCs to that info post? There's actuallt now also the T5s but I don't know too much about them except the hype and that they are skinner than standard flos.


Since I am a newbie and therefore your target audience, let me ask this:
If PAR is the relevant number, how does one figure out what PAR each type of light provides? Is there a target value for PAR that each type of coral requires? What would be great would be a translation between Watts per gallon and PAR - for instance "X" WPG of PC equals "Y" PAR in a tank of 18in depth, etc...
Just my thought.


I agree don't use the watts per gallon calculations and Lumens are off depending on the light K ratings. Okay I understand that
But what is the rule now. I need to know the photoysnthetic light output from the manufacture?
Does anyone have a formula or rule or something to use? I would like to bulid my own fixture for a 55 gallon tank. I want to run relays and timers for morning sun, sun at high noon, afternoon sun, evening sun, and moon light stuff like that. I know how to do all that but don't have a clue what bulbs to use or what is required for a tank with corals.


Active Member
There really isn't a formula to use, and just to warn you in advance, you are going to have a VERY hard time running light cycles like you describe without a ton of different bulbs. You should just look into a Solar Master or something . . . it does everything you describe.
What type of lights to get depends on what type of corals you want to keep . . . soft? LPS? SPS?


Staff member
If we can come up with a final info-article on Lighting for the 101 series that would be great. Its better not to post conflicting opinions because this is 101 for new hobbyists, and lighting is confusing enough. Let me know.


Active Member
I've made a few changes/additions etc. to the original post.
Bang, I completely agree with your points and have edited my content to reflect them. However the last two points you mentioned regarding Kelvin vs. depth and spectrum I left out because at this point in the game for a new hobbiest I think that those ideas are a bit much.
Thanks for the input! Change whatever you want in the thread . . .


:confused: :confused: So would you or not recommend Metal Halide lighting or not for a newby? I plan on a reef tank on down the line. Do I need to start off with this now or something else?


What other type of lighting would you recommend that is just about at the equivalent but at a lower price? Any name brands or specifics??


New Member
Thank yo so much for sharing your extensive knowledge. I am thinking of starting a SWT. I have recently downsized my freshwater tank and am interested in getting in to SW. Your information has been most helpful and I will post my progress and direct any questions I may have to you!! Thanks Again


Thank you so much for the informative post!!

Since wattage is actually a measure of power required to run the bulb (not output), one would ideally try to find the output they need with the lowest wattage, correct?