Before looking at any types of filtration it is important to understand the difference between biological and mechanical filtration. Both are important and needed in aquariums.

Mechanical filtration: Basically, it is the removal or trapping of large suspended waste in the tank to maintain appearance. Wastes may include pieces of plants, fish feces, uneaten foods, and possibly dead fish. Mechanical filtration physically blocks the larger pieces of waste and has gaps small enough to allow the liquid to flow through.

Mechanical filtration – solid particles become trapped

Biological filtration: Secondly, biological filtration. This type of filtration converts fish pollutants into a less harmful state by the use of other organisms, mainly bacteria. We test the water quality in order to determine how successfully this biological filtration is. Fish wastes are disposed by heterotrophic bacteria to form ammonia, then by nitrosomonas to form nitrite, and nitrobacteria to form nitrate. This is a very basic overview. But it does give an idea of how wastes are broken down. Both ammonia and nitrite are still quite toxic to fish. However, nitrate is the end result of this biological action. This is much less toxic to fish. It too can be broken down by aerobic bacteria in controlled conditions. The easiest method however, is to simply change some of the water. Regular water changes can ensure that nitrate levels stay within the acceptable limit.

Water Temperature

Water temperature is an important parameter for most species. Its important that all fish living in the one aquarium are suited to the same water temperature. Large temperature changes can stress fish so it is recommended that most aquariums be heated to maintain a constant temperature to keep your fish healthy. In large aquariums it may be necessary to run two heaters at opposite ends of the tank to ensure consistent temperatures throughout all the water. Two heaters can also act as a safety measure in case one fails, leaving one working whilst sourcing a replacement. With reef tanks, containing expensive corals, power failures can be of great concern. Rapid drop in temperature coupled with the loss of filtration can lead to full tank wipe-outs. Most keen hobbyists with expensive livestock are equipped also with power generators for back up in case of extended power failures. It takes only a sudden change in temperature of 4 degrees Celsius for livestock to be severely affected, not to mention the possible loss of important bacteria in biological filters. Constantly monitor water temperature and aim to maintain it all year round. I’ve found water temperatures of around 80F or 27C to be the most successful.


Ammonia exits in tanks in two forms: the ionic form (or NH4) and ammonia (or the molecular form NH3 ). The molecular form is the one toxic to fish. The relative amounts of ammonia are dependent on the pH. Remember that basically, the higher the pH, the higher the amounts of NH4 that are converted to NH3. Ammonia levels in marine tanks above 0.3 parts per million (ppm) are harmful. Anything over 1.0 ppm is considered deadly.

Marine pH

In the ocean the average pH is around 8.1, this is also the recommended pH for marine aquariums. Levels slightly above or below are considered acceptable. It is possible however to use pH levels to lower the toxicity of ammonia and nitrite. By lowering the pH the toxicity of these deadly conditions is minimized. A pH of 7.5 will help achieve this. Nitrite Nitrite is very toxic to fish, even in quantities as small as ppm. It is developed as a result of the disposal of fish wastes by bacteria. If a biological filter is being developed no livestock should be added until nitrite levels are zero. It is important to test the levels once the tank is established.

Nitrate – not to be confused with nitrate!

Nitrite is very toxic to fish, even in quantities as small as 1ppm. It is developed as a result of the disposal of fish wastes by bacteria. If a biological filter is being developed, no livestock should be added until nitrite levels are zero. It is important to test the levels once the tank is established.

Nitrate – not to be confused with nitrite!

Nitrate ions are produced as a result of biological action occurring in your filter system. Nitrate accumulates in an aquarium in a ratio to the stock load waste and is a good indicator of water quality. Levels of acceptable nitrate are different in aquariums for fish culture only, h’om those containing a reef system.

For a reef system much lower levels of nitrate are acceptable. A level of 5 ppm or lower is optimum. For this reason, reef systems generally have smaller fish and fewer fish numbers. Tests should occur weekly.

For systems culturing fish, acceptable levels are around 20 to 40ppm. Most marine fish will tolerate higher levels around 90ppm without any effect on their health.

Water changes are the easiest way of lowering nitrate levels. It is recommended that 25% of total water amount per month be changed. If more than this is required, then there is a strong possibility your tank is over stocked. Constant water changes stress fish and can lead to their death. Plants and algae can help lower nitrate levels also.


The correct salt content is vital for the health of captive marine species. We measure the salinity or salt content by measuring the actual specific gravity of the marine aquarium. The safest zone for marine creatures is between 1.020 1.025. If the salt content is too high, being greater than 1.025 then we add fresh water until within limits. We need to add salt if the specific gravity is below 1.020. We use an aquarium hydrometer to test the salinity and these are available at all aquarium outlets.

When tank water evaporates the salinity can only become greater. Salt cannot evaporate so be sure to add only fresh water unless test results show otherwise.

Test Kits

*insert test kit*

Air and Water

Part of water quality is diffused oxygen. There are several simple ways to ensure oxygen is maintained in aquarium water. The use of venturis on internal pumps works very effectively. They use water movement from the pump to draw air via tubing into the water. Good quality venturis produce massive amounts of very fine bubbles that are easily diffused. Trickle filters mix and turn the water and diffuse it over bio media whilst operating. They also allow the mixing of air and water. Protein Skimmers are another plus. They work simply by producing millions of tiny air particles at the bottom of a cylinder and rising them in the opposite direction to flowing water. The tiny air particles have a massive amount of surface area that mixes with water.

Although this is not their intended design, they do help aerate the passing water whilst effectively filtering it. Basically anything that promotes air and water mixing is a good thing, the finer the bubbles the more effective it is.

Types of filters

Biological filters
There are many different types of biological filters. All attempt to use bacteria to convert fish wastes into less toxic forms. Some are very simple and easy to make yourself. Some common types are listed below:

– Trickle filter system
– Fluid bed system
– Canister filters
– Bio Wheels
– Undergravel

Trickle filter system
Trickle filter systems are probably the most common biological filtration system for marine tanks. Probably because once this system is established, it is one of the most effective biological filters for hobby aquariums. For beginners this system may seem a little expensive and over the top. However, with a well-designed trickle filter your aquarium is more likely to succeed the first time.

The system consists of another separate filtration tank usually below. The smaller filter tank is filled with bio-media in which bacteria grow. Aquarium water is diffused or trickled over the bio media and collected in the base of the tank or sump. The collected “treated” water is then pumped back into the aquarium to be recycled again. The bio-media is anything that allows or promotes bacteria to grow. The larger the surface area of bio-media the greater the amount of bacteria and hence better filtration. The water usually enters the filter tank by being gravity feed from skimmer boxes from within the aquarium.

It can be dispersed over the filter media by sprinkler wheels or simply plastic tubing complete with holes. The most important thing is that the water is dispersed over as much of the media as possible. An internal or external pump is used to pump the water back into the main tank. The water level of the tank is controlled at the sump. The amount of water that returns to the filter tank is directly proportional to what the lift pump supplies. The skimmer boxes allow water to over flow inside the aquarium and return to the filter tank. A simple system that is very effective.

A good trickle system turns 1.5 times the aquarium volume per hour. Bio-media can be purchased as bio balls or blocks that provide increased surface area for bacteria. They are usually plastic and last forever. You can often purchase second hand bio media from the “Trading Post”, just be sure to wash it with hot water to lessen the chance of disease.

Some points to consider with trickle filters

1. These filters must be planned before the tank is built. Skimmer boxes and return holes will need to be added.

2. They allow the water surface to be skimmed, removing oils and other pollutants from the top of aquarium surfaces. Oils are commonly extracted from fish used as food eg, mullet, pilchards, and yellow tail.

3. As a system they are fairly expensive with HR pumps, plumbing, bio media, and skimmer boxes needing to be purchased. However, once set up are very cost efficient only requiring cleaning periodically.

4. They must be watched for back siphoning. If the power is turned off for any reason, the water in the plumbing can cause a back siphoning effect. It will empty the aquarium until the first outlet hole. You can stop this with one-way valves or simply by drilling a hole at the running water level in the plumbing. When the power is off the tank will only allow the water inside the plumbing to back siphon. Obviously, this hole will act as a return hole while the tank is running and should be kept clean. A good size hole will be less prone to blocking.

5. They allow the aquarium’s water level to stay constant; the sump of the filter tank is the only level that changes.

6. The lift pump for the filter tank can be internal (inside sump), or external. Internal pumps are much cheaper and simple to install. However, the filter tank must be pulled down in order to maintain the pump. Some pumps need to be cleaned every 3 months, so keep this in mind. External pumps are easy to work on and there is no need to disturb the filter tank. However, they require extra plumbing and are usually more expensive. I have had both, and personally, I would go external for the ease of cleaning. Pumps that are hard to access soon become forgotten and cleaning is neglected. Remember this system runs constantly and needs a pump designed to do so. Also check power usage as this may save your power bill in the long term.

7. Trickle filter systems offer a sump in the filter tank that can be used to branch off other filters and accessories. That way all plumbing is kept out of sight and neat. Other filters and accessories include, protein Skimmers, fluid bed filters, canister filters. 8. As water is diffused over bio media it is also aerated. This is an added bonus to water quality.

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