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Lighting for
Freshwater Aquariums

Freshwater Planted Aquarium at Steinhart Aquarium

By Tony Griffitts

There has been a revolution in aquarium lighting in the last couple of decades that the marine side of the aquarium hobby has been quick to embrace, but the freshwater side appears to be lagging behind.  Perhaps it is because of the misunderstanding, misconception, or just plain old bad information floating around on the freshwater side of the hobby that is making it slow to embrace modern aquarium lighting systems.  The aftermarket lighting systems that reef hobbyist have embraced for years have proved to be very beneficial for coral and marine fish. These lighting systems can also provide the same health benefits for freshwater aquariums.  These bright modern lighting systems can be found in many types and configurations.  The most common aftermarket lighting systems today are Compact Fluorescent, T5 High Output (T5 HO) Fluorescent, HQI Metal Halide (double ended), HID Metal Halide (socketed end), and LED (Light Emitting Diode).

Bright light over a freshwater has health benefits for the inhabitants.  Bright light stimulate plants and algae to take up harmful nutrients and produce oxygen.  Ammonium and Nitrate are well know plant fertilizers that are constantly being produced in the aquarium.  A bright light over a well planted aquarium will help take up the nutrients as they are being created.  The benefit is very little to no Nitrate accumulation in the tank.  Since Nitrate will lower the pH as it accumulates in the system, a brightly lit aquarium will normally have a much more stable pH.  Excessive amounts of Nitrate are linked to Hole in the Head disease (HITH) in large freshwater cichlids and Head and Lateral Line Disease (HLLE) in many marine fish.  Keeping Nitrate under control is very important in maintaining a healthy aquarium eco-system.  If you have any Nitrate accumulation in the system over the course of a month or two, your tank is out of balance.  Most freshwater hobbyist do a regular water change on their tank (that helps reduce the amount of Nitrate that is accumulating) without understating the reason why they are necessary.  The number one health benefit from water changes is the reduction of Nitrate on a system that is out of balance.  Adding a bright light over a tank with fast growing aquatic plants can have a significant impact on reducing Nitrate accumulation in the aquarium.

For freshwater hobbyist with large cichlids like Oscars that like to redecorate, and fish that like to eat plants, keeping plants in a brightly lit aquarium would not be practical, but you can still achieve the benefit by circulating the water through a refugium.  Refugiums are very popular in the reef side of the hobby.  A refugium is a separate tank with a bright light that is set up with algae (in the case of the reef hobby) or with fast growing plants (in the case of the freshwater hobby).  Refugiums are set up much like a trickle filter (a.k.a. Wet/Dry filter) normally located underneath the main display tank.  Water is brought to the refugium by either a hang on the back overflow system or by a built in overflow system in the aquarium.  A small sump pump is placed in the refugium to return water to the main display.  You do not need to cycle water very quickly through the refugium to achieve the benefit of Natural Nitrate Reduction (NNR).  The refugium can also be used to house freshwater shrimp that help control algae, or other small freshwater fish that would be eaten if they were placed in the main display.

10 gallon flat back hex with green water.

Green water is caused by a single cell algae that is suspended in the water column, thriving on nutrients in the aquarium.

Freshwater hobbyist often worry about adding bright light over their aquarium will cause an excessive algae outbreak.  Algae is a natural part of the aquarium.  Excessive algae growth is not caused by bright light, it is caused by excessive nutrients.  Algae growth will often be excessive in the beginning, until the plants have established themselves, and then start out competing the algae for nutrients.  It is always best to start off with fast growing plants or add fast growing plants when the aquarium has an algae problem.  Green water algae bloom will sometimes occur on aquariums with bright lighting systems.  In some cases the algae bloom may go away on its own after a month, but in some cases, the nutrients the algae may be thriving on may be coming from your tap water.  The addition of a small Ultraviolet Sterilizer to the filtration system will eliminate any green water.  Once the bright light system is well established with plants the algae will no longer be a problem.  Many freshwater hobbyist with brightly lit planted tanks will confess that they rarely have to clean algae off the tank walls.  This is a fact, when the nutrient level is low, there is very little if any algae growth.  In many cases, you may find that the addition of aquatic plant fertilizers may be necessary to keep your plants growing well.

Bright light also helps enrich the color of the fish.  Bright light helps darken the pigments of many fish, making them stand out.  Discus hobbyist; don't buy into the myth that discus don't like brightly lit tanks.  They may need a few days to acclimate to the brighter light, but in time they will settle down, their colors will start to become even brighter, they will grow quicker, they will spawn, and their water quality will improve.

The addition of a CO2 (carbon dioxide) system will speed up the rate at which plants will take up nutrients, and their rate of growth.  While CO2 systems are not necessary on a brightly lit system, they can help speed up the uptake of nutrients in a system that is out of balance.

When selecting a new lighting system for your aquarium there are few things you need to consider.  Many of these modern lighting systems produce a lot of heat, some more than others, with the exception of LEDs.  Having plenty of ventilation is very important so you do not transfer to much heat into the aquarium.

Two 150 watt HQI Metal Halide lights.

Two 150 watt HQI Metal Halide lights over a 30 gallon tube tank.

Metal Halide

Metal halide lights both HQI and HID get extremely hot, and can give you a 3rd degree burn in no time if you were to touch it after it has been on a few minutes.  You must be able to provide lots of ventilation in the form of auxiliary fans to help kept the tank from getting to warm.  If metal halide lights are mounted in a canopy, you must also mount auxiliary fans.  Metal halide bulbs require special handling when replacing the bulb, never touch a bulb with you bare hands, always use a clean cloth to handle the bulbs.  Oils on your fingers can damage metal halide bulbs.  Metal halide lights above an aquarium make a stunning display.  With surface agitation, metal halide lights give the tank a shimmering light effect similar to the sun shining through a crystal clear lake.  Normally one 150 or 175 watt light is good enough to cover a 24" x 24" (60 cm x 60 cm) footprint with a depth of 24" (60 cm) or less.  If your tank is 30" to 36" (75 cm x 90 cm) tall, you may want to consider a 250 watt light.

Three 55w CFLs over a 60 gallon cube.

Three 65 watt Compact Fluorescent Lights over a 60 gallon cube tank.

Compact Fluorescent

Compact Fluorescent Lights (CFL) are very popular choice with aquatic plant hobbyist.  While they can produce a lot of heat, they are much cooler than metal halide lights.  In some cases you many need auxiliary fans with these lights, especially if they are mounted in a canopy.  These bulbs vary in wattage, depending on the length of the bulb.  For a standard US 10 gallon 20" (50 cm) long tank the bulb is usually 36 watts, for a 24" (60 cm) tank they are either 55 or 65 watts, for a 36" (90 cm) tank they are 96 watts.  I normally recommend one or two bulbs per 12" (30 cm) of width of the tank.  CFLs are generally good enough for tanks 24" (60 cm) or less in height.

T5 Retrofit.

T5 HO retrofit light.

T5 High Output

T5 High Output (T5 HO) Fluorescent are becoming very popular in the hobby.  T5 HO lights are very compact, put out a lot of light, and are cooler to operate than metal halide.  They may require auxiliary fans with these lights, especially if they are mounted in a canopy.  These bulbs vary in wattage, depending on the length of the bulb.  Bulbs for a 24" (60 cm) long aquarium are 24 watts, for a 36" (90 cm) tank they are 39 watts, for a 48" (120 cm) tank they are 54 watts, and for a 60" (150 cm) tank they are 80 watts.  I normally recommend two to four bulbs per 12" (30 cm) width of the tank.  T5 HO are generally good enough for tanks 24" (60 cm) or less in height.

Catalina Aquarium 15 Inch LED Light

Catalina Aquarium's 15 inch LED light system.


LED (Light Emitting Diode) are the newest lighting system for the aquarium hobby.  One advantage these lights have over the others listed above is they produce very little heat.  They are also the most expensive option right now.  They are gaining popularity and their application over nano tanks may be the best option.  Catalina Aquarium (a sponsor of Aquaworld Aquarium) offers a 15" LED fixture that has 180 LEDs that use only .3 watts each for a total of 54 watts.  They say it puts out the same amount of light as a 175 watt metal halide.  These lights are an option if you don't want to deal with the heat, and are willing to pay the extra cost.

The color temperature of the bulb can make a difference on how well your plants will grow, and take up nutrients.  For freshwater the ideal bulbs fall in the 5500K to 6700K range.  10000K bulbs will also provide fairly good growth and appearance.  Higher K bulbs will normally look bluer and this often makes green plants look pale yellow.  Actinic lights should be avoided, as they make plants look yellow.

Before you add a new lighting system to your tank, it is recommended that you do several water changes to try and reduce the amount of nutrients in the water.  This can sometimes prevent excessive algae growth.  Place your light on a timer so the lights go and off even when no one is a home.  Now let there be light!

Published - 20090810

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