Aquarium Lighting Guide

Lighting an aquarium can get a bit confusing pretty quickly. There are tons of light fixtures out there, tons of different types of bulbs, different colors, different ways of describing color, and even more than all this are the opinions.


Color seems simple, but it gets confusing fast. To begin with, some bulbs are described with names such as Plant, Colormax, coral, actinic, daylight, etc. The most precise way to measure color is with the kelvin color. You will see almost all bulbs have a kelvin rating, such as 6,500K. If you remember your science classes, you may remember that in addition to Fahrenheit and Celsius, kelvin is another scale for temperature. So what does that have to do with color? The definition of kelvin color is: “the color of light a black object would give off if heated to that temperature in kelvin”. So a 6,500K bulb isn’t 6,500K itself, it gives off the color a black object would if heated to 6,500K. Don’t worry, this doesn’t make sense to me either, but once you get used to the kelvin color spectrum, it is easy to work with and use to discuss lighting color. Natural sunlight, pure white light, is 6,500K. Below that, the overall color is redder. Above that, it is bluer. You may see many plant bulbs in the 3,500K to 6,500K range since plants absorb more red, so they grow better under lights with more red. Most reef bulbs will be in the 10,000K to 20,000K range, with most people preferring 10-14,000K. In most cases, you won’t be able to simply look at a tank and guess the color. If it is very blue, it is probably 18-20,000K. In most cases, though, it is relative. If you had a tank with an 8,000K bulb next to a tank with a 6,500K bulb you will notice the 6,500K is redder and that the 8,000K is bluer.

Actinic light is a little different because it isn’t a kelvin color. It is above 20,000K and limited to pure blue in a specific range. Actinics are comparable to black lights and if you have anything that is blacklight reactive, it will glow under actinics as well. Actinics are usually used for reef tanks because so many of the pigments in corals glow under actinic lighting, creating a stunning display. Actinic bulbs can be fluorescent or LEDs. They are never used alone, they always supplement white light. They may be on alone for a short period before and after the white lights are on (and they will be on while the white lights are on). The light intensity and color of actinic light help corals produce and maintain their bright colors.

Even on freshwater, I prefer a reef color mix. My favorite standard fluorescent bulb is Zoo Med’s Reef Sun which is a 50/50 bulb (half 6,500K and half actinic). This produces a bright, clean, balanced white light that shows off warm colors as well as cool blues and purples. Most typical freshwater bulbs are too red/yellow for my preference (such as 6,500 alone). As an alternative to the 50/50 type bulbs, I prefer something around 10,000K for freshwater, and half 10,000K and half actinic bulbs for a reef.

Be aware that not all bulbs of the same kelvin color are the same. One brand’s 6,500K may look redder than another brand’s 6,500K. This doesn’t make any sense, but that’s the reality.

Types of Lights


Fluorescent bulbs used to be the most common so we will start there. Fluorescent lights are described by tube diameter and intensity. Tube diameter is measured in 1/8″ and preceded by a T. So a T8 is 8/8″, or 1″ in diameter. This was the most popular and the standard in light strips and hoods that used to come with almost all tanks (most are now LEDs). The narrower tubes are T5 which are 5/8″ in diameter. Although T5s come in a standard output, you will almost always find them as high output designated as HOT5 or T5HO. T12 bulbs are 12/8″ in diameter, or 1.5″. They are rare, but you may come across a reference to VHO T12s (very high output). These are a bit outdated, but one benefit to them is that the bulbs themselves have a built-in reflector (they only emit light from one side). This helps a lot in getting all the light produced actually in the tank.

Although they used to be the norm, LEDs have replaced fluorescents as the standard. Fluorescent lights need to be replaced every six months even if they still work because they get weaker and the color they give off slowly changes over time. They also use more electricity. See LEDs below for more details.

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Compact Fluorescent

Compact fluorescent fixtures are almost gone but you may come across them. They were considered inferior years ago, even when I had my reef. I used a 4×96 watt 36″ compact fluorescent fixture on my 45 gallon reef, so the light fixture took up the entire top of the tank. It was a very good light. I had all sorts of corals in that tank including LPS, SPS, and even clams (and I didn’t just have them, they did very well). However, the bulbs are almost impossible to find anymore. Anyone still selling them probably got them years ago and still has them because they are trying to at least get their cost on them (not a good business decision). So even if you found a good fixture in working order, it probably isn’t worth buying.

Metal Halides

Metal halides are generally obsolete with the introduction of the best LED fixtures. They used to be the standard for reef lighting because their intensity was unmatched. They could penetrate tall reef tanks in a way no other fixtures could. They were intense because they gave off about the same amount of light per watt as fluorescents, but they had a lot of watts crammed into a tiny little 1″ light filament. However, they are expensive and VERY hot. In addition to all the electricity they burn themselves, many if not most tanks with halides needed chillers as well which use a lot of electricity. They also need to be supplemented (at least with actinic). The bulbs need to be replaced every 12 months. Some reef aquarists are going back to halides, but I think the vast majority of people doing that used halides with success before and have not tried the right LEDs.


Reflectors are a big issue with fluorescent and metal halides lights because they alone can greatly impact how much light actually goes in the tank. A 4-bulb HOT5 fixture with a single curve reflector will have a fraction of the light reach the tank compared to a fixture with a good individual curved reflector. With individual curved reflectors, the reflector actually curves around each bulb and even extends down between the bulbs (even below them in the best fixtures). This can have a massive improvement in light reaching the tank because it is all being reflected down toward the water instead of bouncing around until hitting the other bulbs.


LEDs are the new standard for high-end lighting. There are lots of LEDs out there and they can range from under $50 to well over $600. The biggest benefits of LEDs are that the bulbs never need to be replaced, they use a lot less electricity, they run a lot cooler, and the bulbs actually direct the light straight down so all the light is actually going down into the tank. In addition, the better LEDs also allow you to adjust both color and intensity (and even pretty affordable fixtures have these features, not just the most expensive). Together, these benefits make LEDs the hands-down best option for lighting.

Thankfully, you can now get great LED lights without spending $400-600 each for the top-shelf lights. Radions used to be the best, and they are still outstanding lights, but Kessils are at least arguably better because they mix the light together and it comes out of one lens so you don’t have spots of red, spots of green, etc. In addition, although Radions and others give you amazing customizability with super-detailed programming, many people can’t stand the fact that you can’t just manually turn the light on or off manually when needed, you have to go into the app.

I personally have had great success with black box LEDs. These are non-brand name fixtures (usually made in China) that are much cheaper but can be just as good as top-shelf LEDs. So although they aren’t as sleek and modern looking compared to a Kessil or Radion (and you should be looking in the tank anyway, not at the lights), and you do give up some bells and whistles depending on exactly which two fixtures you’re comparing, they definitely get the job done well. I’ve had everything from softies to SPS and even clams thrive under black box LEDs.

For me, the biggest difference was the actinics. Before I switched to LEDs, I had both compact fluorescents and HOT5s. I had really good fixtures with great reflectors and the best bulbs. When I switched to LEDs, I instantly saw a different tank. The colors were so much more vibrant and intense, I couldn’t believe it. I knew they were good, but I had no idea that even with great fixtures and the best bulbs, I was still missing so much of the color that was already there. It’s something that pictures can’t accurately show, you have to see it in person. As soon as I saw how much better my corals looked, I knew I had been wrong for waiting so long to switch to LEDs.

If you have a smaller tank, up to around 40 gallons or so, something like a Current or even Nicrew will do. But above that, I’d go straight to the black box LEDs. For the price, you won’t be able to beat them and they can provide A LOT of light.

Another thing to note, many LEDs list a range for the tank size they fit. For example, they may say 36-48″. This means the light is only 36″ long. So if you want the light to span from one end to the other, go by the smaller number. If you don’t mind some space on each end, it’s okay to go by the larger number.

Whatever type of LED light you do, make sure you can adjust the color and intensity. This will give you the flexibility to use it on freshwater, planted, or a reef and to adjust the intensity as your plants or corals are ready for more, or as your own preference changes.

Splash Guards and Glass Tops

Once you have all this great light coming out of the fixture, you can keep a lot of it from getting to the tank just by neglecting the splash guards and/or glass top. Glass tops are easily neglected and any algae, mineral deposits, or salt creep blocks light. I have seen tanks look like they got new bulbs and even new fixtures just by cleaning the glass tops. In fact, it is even possible to burn corals by the sudden increase in light after cleaning neglected glass tops. If you aren’t using a glass top, then you definitely want to keep the splash guard on the light fixture (you don’t want bare bulbs over open water). Most HOT5 and other higher-end fixtures come with splash guards. If you have a glass top too simply remove the splash guard. If you are using the fixture over open water, keep the splash guard on and well cleaned.

While we’re on the topic, these fish are dragged out of the ocean, left to overheat on a dock, shipped halfway around the world, re-bagged, shipped halfway across the country, get crammed into a retailer’s tanks with fish they can’t get away from, are forced to start eating bizarre new foods or else starve to death, get taken home where they get a whole new set of tankmates they can’t get away (and in tank that may or may not even be close to an ideal home for them), half of the fish die during all this, and the ones that actually make it we’re putting at even more risk by rolling the dice and risking them jumping out to end up dried up on the floor? Keep your tanks covered. Glass is fine. Some people don’t like it because it blocks some light, but if your lights can get light through glass, you need a light upgrade anyway. At the very least, get the clear mesh tops that let air through but will keep fish in.


The amount of time that lights are on is another major issue with lighting. In general, I prefer to run any lights on a timer. This way there are no issues with the photoperiod. You never forget to turn them on or off or let them run way too long. There are tons of timers out there. I do not like to use fixtures that have built-in timers, they can be complicated to use and can make it much harder to just turn the light on or off outside of the normal schedule. Any of the light timers made for aquariums or found in any hardware or department store (such as Walmart or Target) should work. Most light fixtures require a three-pronged outlet so a three-pronged timer is required. The only other thing I want to make sure I have on a timer is a switch so that I can turn the lights on or off any time I need to.

Fish need a day/night cycle. In a fish-only tank, the light from the room is enough to provide this unless the tank is in a dark room with no sunlight. Most of us want actual light to enjoy the fish. In fish-only tanks, this can be as little as a few hours in the evening when you are actually home to enjoy the tank.

Reef and planted tanks are a little more complicated since they do require a certain minimal amount of light. Most planted tank fixtures are pretty simple so you just have the lights run 6-14 hours per day, as much as you can without algae problems. Reef tanks usually need a shorter photoperiod to avoid algae issues.

In addition to total photoperiod, reef tanks should have separate timers for the white and the blue lights. Most people run the blue lights an extra hour on each end that the whites are on. For example, if the white lights are on from 4pm until 10pm, the blues will usually be on from 3pm until 11pm. This gives the corals and fish a dimmer light than just suddenly being in full light. It also gives you a little time to enjoy how good the corals look with just the blues on.

Be aware that you cannot compensate for relatively weak lights by having them on for a longer period of time, it just doesn’t work. So if you are trying to have a planted or reef tank with something like a 2 bulb HOT5 or an LED fixture that’s weaker than it should be and it just isn’t enough light, you need a better fixture, adding more hours of light just won’t do it. In fact, in some cases, it can promote algae problems since the tank isn’t balanced.

Light Distribution

One often overlooked factor is light distribution. I’m not talking about how well a particular fixture spreads out the light it provides (which can be very important), I’m talking about how evenly you light the whole tank. If you have a 48″ tank, you will almost certainly get a 48″ fixture (or equivalent pendant lights). But, if it isn’t a reef or planted tank, it can create a really great look to not light the whole length of the tank.

I have a 220-gallon with silver dollars, a bichir, some catfish, and some small to medium cichlids. Although it’s a six-foot tank, I only have light on one end. This creates a really interesting look to the tank because one area is well-lit, one area is lit, but not directly, and at least 1/3 of the tank is dark, especially since the unlit section also has a giant piece of driftwood in it that blocks even more light.

This not only creates a unique and interesting look for the tank, but also lets the fish feel much more comfortable. Many fish we keep are nocturnal or at least would not be out in the open in direct light. By providing varying light levels, the fish can feel much more comfortable. You may think this would cause them to hide more, but it can actually have the opposite effect. If a fish wants a dark spot and the whole tank is bright, that fish isn’t going to leave that one dark spot in the tank under a piece of driftwood, rock, inside a decoration, or wherever that dark spot is. On the other hand, if whole sections of the tank are dark and shadowed, that same fish is much more likely to be comfortable enough to come out and hang out in the dark end of the tank.

Even in a planted or reef tank, if done properly, only lighting part of the tank can look really nice. Having high and low light areas can allow you to place corals or plants where they will do best. And even having a completely dark end can allow the fish to feel comfortable and provide a whole new, unique dimension to the tank.

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Six-foot 220-gallon Aquarium with only one light creating a bright open area as well as a dark sheltered area.

Other Articles You May Be Interested In

Introduction to Reef Tanks

Introduction to Corals

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