Marine Aquaria/Marine Science 101

To be a smart reefkeeper and make intelligent decisions, it is quite helpful to have a good understanding of some of the science involved. This section will give an introduction to some of the science to help give a better understanding to reefkeeping.

Water ChemistryEdit

When you look at clear water, do you ever stop to think about what's in it? Even that bottled distilled water or spring water isn't pure; there's usually something else in there. Having a good understanding of what's in water will help you understand better the environment within which aquatic organisms inhabit.


pH is a variable that measures how acidic or basic (also called alkaline) a solution is, in terms of Hydrogen ions (hence the H in pH). The deep science of pH is not particularly important, but an aquarist should at least understand the basics (pun not intended), as the pH of the environment has a great effect on the organisms.

pH is a scale usually from 1 to 14. The lower the number, the more acidic. Conversely, the higher the number, the more basic. A pH of 7 is said to be neutral; this is the pH of pure water. Seawater is approximately a pH of 8.0-8.3, so it is basic.

The pH of a solution can change. If a substance makes the pH become lower, it is said to be an acid. As you have probably guessed, a substance that makes the pH higher is called a base. If your pH is changing in some way, you can bet that some sort of acid or base is playing a role, depending on which the direction your pH is changing. We can use this very same property of being able to change pH to help control it and stabilize it at a pH we want.


Salinity is the measure of the amount of salt in water. Of course, in the marine hobby, salinity is quite important. Salt balance is very important to all organisms; if you've ever seen salt put on a slug, you'll know why. Fluctuations in salinity can be quite dangerous, especially to simpler organisms that are not able to regulate their own salt balance. However, even fish can only take changes in salinity to a certain point.

The salinity of seawater is measured in a number of ways, including percent or ppt (parts per thousand). However, in the aquarium hobby, the most common way to express salinity is by specific gravity, which compares the density of substances to freshwater. Because density is temperature dependent, specific gravity readings are usually given for 25°C. Freshwater has a specific gravity of 1.000, while saltwater is about 1.021-1.025.

One final note is that salt does not evaporate. Therefore, as water evaporates over time from your aquarium, the salt remains, and therefore the salinity of your aquarium will increase slightly. It is important to keep adding freshwater to the aquarium to make up for evaporation.


Hardness is a measure of mineral content. There are two types, General Hardness (GH) and Carbonate Hardness (KH). These systems were developed by the Germans, which is why these acronyms only loosely correspond to the words. If water is high in these dissolved minerals, it is called hard. If the concentrations of minerals are low, it is soft.

General Hardness is a measure of calcium or magnesium ions in the water, and its units are dGH, or degrees of General Hardness. 1 dGH corresponds to 10 mg of calcium oxide or magnesium oxide in 1 L of water.

Carbonate Hardness is a measure of carbonate or bicarbonate ions in the water, and its units are KH. A degree of KH corresponds to 17.83 mg of (usually) calcium carbonate per 1 L of water. The units are in terms of calcium carbonate, but calcium bicarbonate also exists in water commonly, though each ion only counts half as much towards hardness as carbonate does.

Hardness can have an effect on the ability of pH to change; both carbonate ions are effective as buffers to keep pH basic. With a good level of hardness, the aquarium will resist shifting towards acidity. It is also important to keep the hardness steady and high as many shelled organisms such as molluscs build their structures from the hardness in the water. Soft water means that their shells will not be very well developed.

Dissolved OxygenEdit

As most of the organisms we care about keeping are indeed dependent on oxygen, dissolved oxygen is important.

Oxygen moves between the water environment and the atmosphere through the surface. Therefore, the more surface area your aquarium has, the better. Oxygen transfer can be improved even further by water movement.


Nitrogen refers to the nitrogenous wastes given off by organisms. The main ones of concern to us are Ammonia (NH3), Nitrites (NO2), and Nitrates (NO3). Ammonia is the most toxic, nitrites are the second-most toxic, and nitrates are much less toxic. In the marine aquarium, ammonia and nitrite should not be present at all. Nitrates may be present in Fish Only tanks as many fish are not as sensitive to nitrates, but the best would be to have very little or not to have any nitrates, especially in aquariums containing invertebrates.

See more about Nitrogen in the Nitrogen Cycle below.


Phosphates are a necessity of all organisms and are a basic part of many important molecules. However, the problem arises when there are excess phosphates in the water, which may lead to unwanted algae growth. This algae can be unsightly, but it can also grow other surfaces and grow over organisms which you may want. If the algae dies in the tank and decomposes enough, there will be less oxygen available due to the effect of this decomposition.

Total Dissolved SolidsEdit


Nitrogen CycleEdit

The Nitrogen Cycle describes the changes in nitrogenous compounds in the environment. Because many nitrogen compounds are toxic, it is important to know something about this cycle. Luckily, these nitrogenous compounds are converted to less and less toxic forms through this Nitrogen Cycle. We shall simplify the Nitrogen Cycle.

If you start with your organisms, they release a compound known as ammonia as waste product or a product of decomposition. Ammonia is quite toxic and quite dangerous. Luckily for us, there exists a process known as nitrification by bacteria convert these waste products to less toxic forms. These bacteria live in aerobic conditions and benefit from the presence of oxygen. First the ammonia is converted to nitrites by Nitrosomonas; this compound is still toxic. Next, nitrites are converted to nitrates by Nitrobacter or Nitrospira. Nitrates are much less toxic compared to ammonia and nitrite. In an environment with a healthy colony of these nitrifying bacteria, ammonia and nitrites will reach 0.

However, as you guessed it, this leads to a build-up of nitrates. When it reaches higher levels, nitrates can become dangerous as well. The best way to remove nitrates is through water changes. In some aquariums though, another process exists as part of the Nitrogen Cycle to help reduce nitrates.

Denitrification is the process by which nitrates are converted back to nontoxic Nitrogen gas. This is also done by bacteria, but unlike nitrifying bacteira, denitrifying types of bacteria only live in anaerobic conditions without oxygen. In the marine aquarium, this means either a Deep Sand Bed or a Live Rock, both which shall be discussed in further detail later. Both of these house anaerobic pockets that allow these bacteria to survive in your aquarium. They can contribute to removing nitrates from the water.

Plants are also able to take up nitrogenous compounds, removing them from the water. Plants are not generally popular in the marine aquarium hobby, however types of macroalgae are available for this job. In using macroalgae to reduce nitrates, to actually remove the nitrogenous compounds from the aquarium means to "harvest" the algae every so often.

Carrying CapacityEdit

No matter what the environment is, there is a limit to the number of organisms that can live there. In ecology, the maximum number of animals a region can maintain is called the carrying capacity. This holds true for aquariums as well.

You cannot put too many animals into too small of a space. Eventually, the point will be reached where the animals produce too much waste or use up the resources available. In the aquarium, this usually shows itself as producing too much waste, as resource availability is less of an issue as food and such can be added daily. When too much waste is produced, the tank will be polluted and certain other organisms, usually unwanted, may start to grow, such as algae.

The environment is also limited by the amount of physical space available. You can only squeeze so many animals into a space before they begin to feel uncomfortable from not having enough room. This is especially true for fish, and less true for some of the invertebrates such as corals, which in their natural environment often live relatively close to each other. By not having enough room to itself or not having enough room to swim, an animal may become stressed. Some animals may be territorial, reducing the number of other animals that can be kept in the aquarium.