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 best 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 the blue end (almost entirely blue and violet). 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 at night or for a short period before and after the white lights are on (and they will be on while the white lights are on too). The light intensity and color of actinic light help corals produce and maintain their bright colors.

The pigment in corals is protection against UV damage. It is basically how they tan. Humans tan brown, corals tan neon orange, neon green, etc. Most stores have very intense lights that are heavy on the blue and actinic. This keeps the corals very bright for sale. Too many aquarists have relatively weak lights. Most corals will brown out pretty quickly after coming home. You may not notice it until you go back to the store and see other corals again that are brighter than the ones in your home tank. Corals actually grow faster under redder light since red light is needed for photosynthesis. The corals themselves are not photosynthetic, but the algae that live inside them are. Since the bright colors are protection against UV, if your lights are much brighter than those at the store, or you place a coral high up close to the lights, you can burn your corals. This can cause them to not open (as can too much flow, depending on the type of coral), bleach, and burn. It is better to start them low and move them up than risk burning since it is easy to undo browning, burning can be lethal.

Even on freshwater I prefer a reef type 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, 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 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 are 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 is the most popular and the standard in the 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.

In most freshwater tanks, standard fluorescent lights will be adequate. There are lots of fixtures out there, most are single tube, but you may come across a dual/twin bulb fixture, and at least one company made a triple fixture that held three tubes.

Fluorescent bulbs use a moderate amount of electricity, can be pretty warm if not hot, and need to be replaced every 6 months. However, if you are doing a planted or reef tank you will probably be looking at something along the lines of a HOT5 with at least four bulbs, in which case you have a lot of control over the color of the bulbs and can mix and match as desired to get the result you want. Many fixtures have different switches and even cords so that two bulbs can be on one timer and two on another. This is ideal on a reef where you will want the actinic bulbs on before and after the whites.

In general, I prefer fluorescent fixtures. I have had great success with them over the years including my planted 75 gallon goldfish tank that had nothing more than two standard fluorescent bulbs on it (even I don’t believe the tank did so well on that little bit of light). UPDATE: I now prefer LEDs. The cost savings from not having to replace the bulbs every 6 months alone make them worth it. In addition, with most fixtures you have at least some if not A LOT of control over the exact color balance and intensity.

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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.

LEDs are the new standard for high end lighting. There are tons out there and can range from under $50 to well over $600. LEDS stay cool/warm, use very little electricity, and have a great light output. Some fixtures are better than others about being able to control the color and intensity of the light. Some even have the ability to mimic natural lighting conditions, going as far as having fake flashes of lightning during a ‘thunderstorm’. In my opinion, this isn’t necessary at all. Today, even fixtures under $50 can provide easy control over the exact color and intensity. LEDs are definitely the new standard in lighting and I wouldn’t use anything else anymore.

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, but they had a lot of watts crammed in to a tiny little 1″ light filament. However, they were expensive and VERY hot (many if not most tanks with halides needed chillers as well). They also needed to be supplemented (at least for actinic).

The light from metal halides is very bright and the bulbs last 12 months. They also provide a shimmer effect, something most people like, especially in a reef tank (although you can get this with pendant LEDs too). The heat and electricity consumption are significant issues though. So unless you find a real steal of a deal on craigslist or something and know the heat won’t be an issue, you are better off going with 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, not letting the light simply reach the bulb next to it.

Reflectors aren’t an issue for LEDs since each LED bulb is designed to send all the light downward. Most fixtures will even list the angle of the light spread of the LEDs.

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 dirty 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.


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 any built-in to power strips or light fixtures, but any of the light timers made for aquariums or found in any hardware or department store (such as Walmart or Target) are fine. 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 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 there to enjoy them. The most natural photoperiod would be 12-14 hours of lights on during the day, but in many tanks this would cause too much algae.

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 8-14 hours per day, as much as you can without algae problems.

Based on having a HOT5 fixture reef lighting has 3 parts depending on the fixture: the LED moonlights, the actinic bulbs, and the white bulbs. The moonlights can run 24 hours per day or for an hour before and after all the other lights are on. The actinics can come on an hour before and stay on an hour after the white lights. The white lights can run 6-14 hours per day depending on how intense the lights are and how well the tank is running. If the lights are very intense they can more easily cause an algae problem. Tanks that are very well balanced will have minimal nutrients available for algae problems. Ideally reef tanks have a refugium which is a compartment (usually in the sump) where macroalgae is grown as a way of using up nutrients so that algae in the display tank doesn’t have enough nutrients available to it. This helps keep nutrients and therefore algae under control.

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 and it just isn’t enough light, you need a better fixture (such as a 4 bulb HOT5), 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 an interesting look for the tank, but also lets the fish feel much more comfortable. Many fish we keep are noctural 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 ot 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.