Basic Saltwater Fish Tanks
Biological Filtration
Undergravel and Wet Dry Filtration

You can set up a basic saltwater aquarium using an undergravel filter for basic biological filtration. Start by placing crushed coral or puka shell on a premium undergravel filter plate. Operate the filter plate with powerheads designed to pull water through the filter plate. Eventually, after several weeks, the surface of the crushed coral or puka shell will become live with bacteria that will help to filter the aquarium water. In addition to the undergravel filter, other types of mechanical filters can be used to clarify and improve the water quality in the aquarium. In conjunction with the undergravel filter a quality protein skimmer, backfilter, or canister filter should be considered. Using these optional mechanical filters along with an undergravel filter will help to further balance the saltwater aquarium. Because powerheads circulate water more effectively than standard air pumps, Animal Attraction recommends to operate the undergravel filter with powerheads. Also, you should place at least 2 ½” of crushed coral on top of the undergravel filter. Correct waterflow and substrate depth will eventually cause this system to provide adequate biological filtration. A system with biological filtration will require routine maintenance:


  • a 25% water change every 2 weeks is recommended
  • vacuuming detritus from crushed coral while performing water changes
  • proper maintenance of the mechanical filters being used
  • change 30% of crushed coral or shell about once a year


note: because this system is not considered a NATURAL LIVING filter and does not effectively remove nitrate gases. Animal Attraction does not recommend this system for keeping corals and anemones. Undergravel filtration works well for keeping fish if the water quality is maintained by performing routine water changes.


For basic saltwater aquariums larger than 50 gallons one might consider a wet-dry filter system that can be placed in a cabinet underneath the aquarium. This effectively could take the place of an undergravel filter system or other mechanical filters. Wet-dry filtration can even be used in addition to the other mechanical filters mentioned above. (especially protein skimmers)


note: wet-dry filtration, undergravel filtration, bio-wheels and other types of biological mechanical filtration works well for saltwater fish only aquariums. These filters remove ammonia and nitrite gases very effectively. However they do not remove nitrate gases. Low levels of nitrate gases are acceptable in fish onlyaquariums as long as they are kept low by performing routine water changes. Because it is important to keep nitrate gas levels very low in aquariums housing corals and invertebrates this would lead Animal Attraction to recommend other types of filtration for aquariums housing coral, anemone, and fish combinations.


Natural Living Filtration


Certain types of filtration utilizing denitrifying bacteria and also algae or plant life that converts nitrate gas into dissolved oxygen and free nitrogen are considered to be NATURAL LIVING FILTER SYSTEMSThese natural living systems provide a more complete and balanced approach to filtering and maintaining a marine environment suitable for both fish and corals. There are many different types of natural living filters that are becoming overwhelmingly preferred and recommended in the hobby today. (note: these different types of natural living filters can be used effectively on their own or even more successfully in combination with other types of natural living filters and mechanical filters.) Some different types of natural living filter systems are listed below.




A deep sand bed will house huge colonies of denitrifying bacteria. Oolitic aragonite sand also slowly releases natural elements into the marine aquarium. This provides for a balanced type of natural living filtration and a steady buffer to help maintain calcium, alkalinity, and P.H. levels. To get these benefits one could start with a 4” + deep bed of oolitic aragonite sand placed directly on the glass bottom of the aquarium. Once the saltwater aquarium is filled with water and the correct salt mixture is added then the sand should be seeded with LIVE SAND. (If you choose crushed coral or shell instead of aragonite sand, then we recommend less than 1” on the bottom. However, a deep sand bed is proven to filter much more efficiently than a crushed coral bed.) To calculate the amount of fine sand needed to make a sand bed that will filter properly apply the following formula using inches.


Width x length x depth of bed desired x .059 = pounds of sand required




Live rock houses large colonies of denitrifying bacteria, detritivores, macro algae, and many other beneficial organisms. Ample amounts of live rock will help to balance and filter a marine aquarium. Place around 1 pound of live rock per gallon of water in the aquarium. Try and stack the live rock with caves and crevices and also try not to have too much of the rock touching the sand bed. This will allow for good water flow and also let the sand bed filter more effectively. A good quality protein skimmer, backfilter, or even a canister filter will provide extra mechanical filtration for the system. Good mechanical filtration in addition to a sufficient amount of live rock can prove to be an excellent start toward balance and filtration for your marine system.




Submersible pumps or powerheads can create extra water flow for the system. Producing alternating waterflow (mixing flow in many different directions and speeds) will help to reduce build up of detritus on live rock and help to keep dead spots from forming on sand beds. Areas on sand beds and live rock that have little or no flow will be prone to growing problem type algae and collecting layers of detritus. (detritus = physical waste, fish poop, decaying food, etc..)


Return pumps from refugiums and sumps can also provide excellent sources of water flow in the system. If a sump or refugium is being utilized with the aquarium, then it is recommended to turn the water over in the system at least 10 times per hour. This means in a 50 gallon aquarium you should have at least a 500 gallon per hour pump.(10 x 50 = 500g.p.h.) Keep this in mind when purchasing return pumps.




There are also many new innovative devices for creating alternating water flow in the main aquarium. Cycling timers for pumps, wavemakers, rotating return nozzles, spray bars, and many other devices will help to create alternating water flow in the aquarium. Try and keep this in mind when setting up your show tank especially when housing corals in your aquarium.




Cleaning crews are very important in helping to control algae, detritus, and naturally move the sand bed to keep it filtering correctly. Cleaning crews consisting of an appropriate mixture of certain types of hermit crabs, algae eating snails, detritus eating snails, detritus eating sea cucumbers, some urchins, select starfish, and even many types of fish species are required in maintaining a healthy and balanced system when utilizing sand beds or live rock for your filtration needs.


A typical mixture of cleaning crew should consist of about 1 or more cleaners per gallon of tank water or per pound of live rock. So if you have 50 lbs. of live rock in your aquarium then you should attempt to house at least 50 mixed cleaners in your aquarium.




  • 15 scarlet reef hermits
  • 5 turbo snails
  • 10 margarite snails
  • 10 nerite snails
  • 10 red band trochus snails
  • 10 nassarius snails
  • 1 tiger tail cucumber
  • 1 blue tuxedo urchin
  • 1 sand sifting starfish
  • 1 rainfordi goby fish


This mixture of cleaners would help to balance a system with live rock and a live sand bed. Understand that some losses of the cleaning crew will occur and it is very important to replace the cleaning crew losses to maintain appropriate numbers of cleaning crew in order to continue to balance that system.




A refugium is a chamber in the aquarium that gives refuge to certain types of organisms to grow and thrive with out being bothered or eaten by the animals that are residing in the main aquarium. Most common refugiums are set up as a sump or filter box plumbed into the main aquarium. Usually this filter is placed in the cabinetry underneath the main aquarium. It is generally recommended to make sure that the refugium has at least 25% of the water capacity of the main aquarium. So a 100 gallon aquarium should have at least a 25 gallon refugium.


Currently most refugiums have a lighting system, a pump for returning water to the main aquarium, a chamber for growing certain types of beneficial algae, a chamber for laying down a live sand bed or mud bed, and many have a chamber for in sump protein skimmers. A refugium can be useful in many different ways.


  • it can be used to grow different types of algae that will help to naturally filterthe marine system.
  • photosynthesis (via the growth of algae) will help to balance P.H.
  • natural planktons can form in the refugium and be pumped into the main aquarium. These planktons can naturally feed the corals and smaller inhabitants of the main aquarium.
  • the refugium can also support a deep sand or mud bed that will add even more surface area for the growth of nitrifying bacteria in the system. Remember these bacteria are good natural filterers.
  • the refugium can also support extra live rock that house nitrifying bacteria.
  • (a great place to hide aquarium devices)a large enough refugium will also allow space for many extras including an in sump protein skimmer, an area for chemical filtration, an area for the aquariums heater, and many other aquarium devices can be stored underneath the aquarium (out of sight) in the refugium.


Adding a refugium to your system has many advantages that will eventually help to balance your entire marine system. Aquariums with refugiums utilizing deep sand or mud beds and also growing algaes that naturally filter the water have proven to be very healthy marine systems. At this point in time this type of natural living filter system seems to overwhelmingly be the method of choice by most advanced hobbyists.




Normal aquarium florescent lighting will suffice for most fish species. Although, if you choose to house corals in the aquarium then your lighting system should be carefully considered. Most coral species require intense lighting to help sustain the growth of the coral. Many corals have a type of algae that live inside of the coral that help the coral to process food into a useful energy source for the coral. The algae is called zooxanthellae algae. If the light intensity is not strong enough then the zooxanthellae algae will die off and in turn the coral will die off also. The general rule of thumb is to have a minimum of at least 5 watts per gallon of aquarium. This will be enough intense light to sustain many popular corals in the aquarium. However, there are many corals that may require even more intense lighting to provide proper coral growth.


There are many different light options to choose from for your marine aquarium. Currently the most common and popular are as follows. There are:


  • V.H.O. lights = very high output fluorescent lights
  • Power Compact fluorescent lights
  • T5 florescent lights
  • Metal Halide lights
  • H.Q.I . Metal Halide lights


The H.Q.I. metal halides and standard metal halide bulbs tend to burn the brightest and most intense. They also tend to cost the most to purchase and also burn the hottest in comparison to the fluorescent type fixture and bulbs. Animal Attraction has always been most impressed with the corals growth rate and survivability when kept under metal halide lighting systems. Metal halide fixtures and bulbs will allow the hobbyist to keep and maintain almost any type of coral preferred. Our #1 choice for the health and balance of your corals and marine system.


Power compacts, T5’s, and V.H.O.’s , are currently the most popular choices of hobbyists when it comes to florescent lighting options. Sleek and stylish power compact and T5 florescent fixtures have recently been entering the market. These light systems have proved to be very effective in keeping many varieties of corals in the aquarium. Although they do not produce as intense of lighting as the metal halide systems can produce (for the wattage used in comparison) they are slightly more affordable and tend not to burn as hot as the florescent options.


Animal Attraction strongly recommends charting the age of your light bulbs. Your bulbs should be changed on a regular basis to provide the proper intensity and spectrum from your bulbs to your corals. The bulbs may lose their spectrum at different rates. Here are the recommendations for changing the different bulbs.


  • Change V.H.O.’s every 6 months
  • Change Power Compacts every 6-8 months
  • Change T5 florescents every 6-8 months
  • Change mogul base standard Metal Halides every 10-12 months
  • Change H.Q.I. Metal Halides every 9 months


Note: many manufacturers will claim that their bulbs will last longer than what we have suggested. We have tested many different brands on our corals and feel that these durations between change outs are in the best interest for the health of our corals and our customers corals.


Also, combinations of the different types of lights can be built into one system. Many manufacturers are building very attractive combinations of metal halides and efficient florescents into single systems. Also, many moonlight arrangements are being built into these fixtures. Marine lighting systems have come a long way in the hobby and are improving by leaps and bounds. Keep up on the latest technologies for your aquarium.


Putting It All Together


A combination of the correct basics will prove to create a health balanced marine system. The important basics are as follows:


  • mechanical and chemical filtration
  • protein skimmer
  • natural living filtration (deep sand bed/mud bed/live rock/caulerpa algae/etc.)
  • a refugium or sump
  • a large enough cleaning crew and a balanced mixture of cleaning crew
  • proper water flow / alternating water flow
  • lighting (intense lighting and proper spectrum of light)
  • correct amount of live rock and correct depth of sandbed
  • biological filtration can be attained with undergravel filters, wet-dry filters, and many other filters including bio-wheel backfilters, and sponges. Reef systems housing corals should have more than just biological filtration. Animal Attraction recommends natural living filtratiion.