I think there are a lot of terms that aquarists new (and not so new) to the hobby may come across and not fully understand the meaning of. I hope the following helps with that. As always, please let me know if there is something you think should be added.

Light that is partially UV. It will look VERY  blue. It has a blacklight effect. This is the light that can make corals look phenomenal. It shouldn’t be used alone, but rather used to supplement white light. (You can turn off the whites to see the corals under just the actinics.)

An environment high in oxygen, or an organism that requires oxygen. Usually in reference to the nitrifying beneficial bacteria that consume ammonia and nitrite.

Algicide or Algaecide
A chemical that kills algae.

An animal that eats algae.

The measure of hardness of water, specifically certain minerals. See KH and GH.

An environment of very little oxygen, or an organism that requires very little oxygen. Usually in reference to denitrifying bacteria that consume nitrate and give off dissolved nitrogen gas.

An environment of no oxygen, or an organism that requires no oxygen. Usually in reference to bacteria that produce toxic hydrogen sulfide gas (usually in bad sands).

Automatic top off (ATO) is a system that automatically adds RO water to a saltwater system to compensate for evaporation. Without it, you have to top off up to twice daily or risk significant and stressful changes in salinity.

Short whiskers on fish such as cories, koi, loaches, etc.

These are antique aquarium equipment you may see referenced in books from the ’80s. They are little plastic balls of cylinders. The idea was that they sat there and water trickled over them. Because the water was only trickling over the bioballs they were exposed to the oxygen in the air (there is A LOT more oxygen in the air than in the water) and therefore the bacteria worked much more efficiently. The problem is that they also trapped tons of debris. As they started to go out of style people who stopped using them only noticed one thing: lower nitrate. The bioballs trapped tons of gunk which then rotted (because it was never removed from the system), which produced tons of nitrate. Bioballs are for trickle filters, not submerged use. Any canister or other submerged filtration method that tries to use them is dead wrong. Don’t waste space in a canister on a biomedia not designed for submerged use.

A thin layer of bacteria that can develop on biomedia, tank decor, and even the glass.

The rate of waste production in the tank. This is affected by the biomass of the livestock, temperature, quality and quantity of food fed, types of fish, and other factors.

The total weight of living organisms. In aquariums, this would be the weight of fish if you put them all in a net and weighed the net, then put them back in the tank and measured the weight of the wet net. The difference would be the actual weight of fish in the tank.

Any biological media that can be used to grow beneficial nitrifying bacteria.

Water where freshwater meets saltwater such as in a bay or estuary. The salinity of a brackish aquarium is usually around 1.008 (about 1/3 that of full saltwater).

An animal that eats other animals.

A compound of chlorine and ammonia that most tap water supplies use. It does not evaporate like chlorine. Many of the tap water supplies that use chlorine will at least use chloramine after severe weather such as heavy rains.

Coralline Algae
Calcium based algae that is some shade of purple and lays down a calcium skeleton base just like coral.

Clean up crew. This usually refers to snails, hermits, etc. in a saltwater tank.

Deionization (DI)
Usually part of reverse osmosis system (see below). This removes ions from the water (positive and negative) and makes RO water even more pure (so pure it won’t conduct electricity). The process can be used on its own, but that usually isn’t used in homes, just big public aquariums.

Active during the day.

Deep sand bed, usually more than 1-2″ deep. This is used to allow for denitrifying bacteria to develop which can consume nitrate. However, it is very easy for a DSB to allow for toxic, anaerobic bacteria to develop which produce hydrogen sulfide. There are better, more effective, and safer methods of denitrification.

A white plastic grid that is used as a light diffuser in 2’x4′ light fixtures such as those in drop tile ceilings. These grids are also perfect for some aquarium uses such as in sumps where they can be used to hold up filter media or as a wall that water can get through but other things can’t (usually used along with plastic mesh/needlepoint canvas). I can only find it at Home Depot. I am sure some other hardware stores carry it, but I haven’t found a Lowe’s that carries it. You can also order it online where you can even get black versions.

Organisms that grow on surfaces. The best examples are things such as sponges, small tubeworms, and other small inhabitants that live on the live rock in a reef tank.

Fish only with live rock. This is a type of saltwater aquarium (basically: no corals).

A small coral. As little as one polyp, but may be many more. This is the usual way to sell most corals (as opposed to buying a whole, intact colony).

See Refugium.


Granular ferric oxide (GFO) is a special type of chemical media that absorbs phosphate and silicate. It is rusty red-brown in color (it is made of rust essentially). It is usually and best used in a media reactor where the slower flow of water through the tumbling media creates the perfect conditions for the removal of these unwanted chemicals.

General hardness. The measure of the overall concentration of calcium, magnesium, and other minerals.

Gallons per hour of water flow.

Plant eater.

Heterotrophic Bacteria
Bacteria that consume waste, debris, and uneaten food in an aquarium. They produce ammonia (fish also give off ammonia directly as their nitrogen-based waste). These are not the bacteria that consume ammonia, nitrite, or nitrate. These are the bacteria responsible for the cloudy haze in a new tank (new tank syndrome).

Hole in the Head.

Head and Lateral Line Erosion.

Hang on back. This usually refers to any hang on back filters but may refer to hang on back overflow boxes and protein skimmers.

An ancient method of testing salinity (specific gravity). This usually uses a lever in the water that floats a certain amount that lets you read the salinity on the scale. These are not nearly as accurate as refractometers and harder to use. Use a refractometer.

Organisms that live in the rock or substrate. The best examples are copepods, small tubeworms, bristleworms, etc. in a reef tank.

Carbonate hardness. The measure of carbonate and bicarbonates in the water. These are what control the pH. A high KH means lots of carbonates and bicarbonates which resist a decrease in pH. They use up the acidity to hold the pH at the same level. It is only when the KH is used up that the pH falls significantly.

Local fish store or local fish shop.

Large polyp stony coral. These are corals that produce a calcium skeleton base and have large polyps. There isn’t an exact group or definition to differentiate large vs small. Examples include frogspawn, brain corals, acans, and many, many others.

A unit of measure of the total amount of visible light.

Complex algae that although not actually plants do look like them. They are usually used in a refugium as a way of sucking up the extra nutrients in the system so that nuisance algae doesn’t have enough nutrients to grow. The best option for this use is Chaetomorpha (key-toe-morph-uh) which basically looks like green steel wool. Caulerpa spp. can be problematic (they can nuke the whole system if they go sexual).

Media Reactor
A special type of filter that holds media in a chamber and pumps water through from the bottom up. Ideally, this means the media tumbles instead of just compacting. This also means it doesn’t collect debris. The ideal flow is low enough to allow more contact time between the water and the media so that the water leaving the media reactor is very clean. The ideal types of media depending on need are GFO, Purigen, and carbon.

Active at night.

Plant and animal eater.

A certain type of calcium-based sand. It is fine and spherical. It is really good at being dissolved so it is a very good way to help keep calcium and alkalinity at the ideal levels. This may be a bit too fine for most reef tanks, but does make an excellent substrate in the refugium.

Photosynthetic active radiation (PAR) is the amount of light available for photosynthesis. I have also seen this described as photosynthesis available radiation, which even if wrong may help you understand it a little better. This is a result of the color of the light and its intensity.

The amount of time the lights are on per day.

Fish eater.

Plankton eater.

Organisms (algae, animals, etc.) that are free-floating in the water column (usually in the ocean) that cannot swim against a current. They may move around in the water but can’t swim independently of the general flow of water.

An individual organism of coral. These can be anywhere from 1-2mm in diameter such as many SPS (acropora, montipora, etc.) all the way up to a foot in diameter (elephant ear mushroom).

Protein Skimmer
A type of filter that runs water and very small air bubbles through a chamber that allows certain chemicals (and even some foods) to cling to the bubbles. The bubbles then build up to a froth in a bubble tower and overflow into a collection cup. This is the only type of filtration that actually removes the waste from the system. Other types just collect it for you to remove. Protein skimmers are generally only used in saltwater because freshwater doesn’t allow small enough bubbles. Freshwater with a good amount of salt (such as some ponds, some goldfish systems, and even some big chain stores’ systems) may have just enough salt to allow bubbles small enough for a special protein skimmer to work.

A special chemical media made by Seachem that has a special resin that removes nitrogen waste products and polishes the water. This is best used in a media reactor, although if you use their special bag or get the 100mL version that comes in a special media bag, it can be used in a canister, HOBs, etc. The media is exceptionally fine which is why you need a special media bag.

A device used to measure the specific gravity (salinity) of saltwater. Pure freshwater is 1.000. Natural seawater is 1.026. A few drops of water are placed on the sample window and you look through the eyepiece. You will see a field of blue and a field of white. The line between the two will correspond to the salinity on the scale visible in the eyepiece.

A safe space. In aquarium use, this means an area protected from other things in the tank. This usually means a compartment in the sump where macroalgae grows (where herbivores in the tank can’t get to it and eat it).

Reverse Osmosis (RO)
RO or RO/DI (includes deionization, see above) is the process of super filtering tap water. It can make it so pure it won’t even conduct electricity. Seawater around a reef is exceptionally low in nutrients. Because of this, algae that can live there is very good at using even very small amounts of nutrients to thrive. Even the small amounts of nitrate, phosphate, and silicate in tap water are enough to turn a reef tank or saltwater tank into an algae farm. Because of this, we use RO water to avoid algae problems and allow corals to thrive. In some rare cases of exceptionally bad tap water, RO may be used in freshwater tanks, but this requires additives to resupply the good stuff (GH, KH, and other good stuff that keeps the chemistry stable and provides minerals). RO systems made for aquariums are much better than ones made for drinking water and produce a truly pure or almost pure product.

The measure of the concentration of salt. This is usually expressed as the reading from a refractometer, so normal seawater would be 1.026 (said as either one point oh two six or ten twenty-six).

Soft Coral
Corals with no calcium skeleton. These include zoanthids, mushrooms, Xenia, and many more.

Specific Gravity
A measure of the density of water. We use it to measure salinity. Freshwater is 1.000. Natural seawater is 1.026.

Small polyp stony corals. These are corals that lay a calcium skeleton base and have small polyps. There isn’t an exact group or definition differentiating large and small. Examples include acropora, montipora, and many others.

A tank that sits inside the stand, under the display, that is used for filtration. It generally has multiple compartments made with baffles that guide the water through different filter media sections. The water is pumped up to the display and drains down through an overflow. If you want to use a sump, build your own sump filter.


T5, T8, T12
The T number on fluorescent light bulbs references the diameter of the bulb in 1/8″. So a T5 is 5/8″ in diameter, a T8 is 8/8″ or 1″ in diameter, and a T12 is 12/8″ or 1.5″ in diameter. This measures JUST the diameter, not the intensity. You can have a normal output T5 bulb and a high output T5 bulb.

Total dissolved solids (TDS) is the measure of the conductivity of water. This is used to measure the purity of RO/DI water. The best RO/DI water is so pure it won’t conduct electricity so it has a TDS of 0. Normal tap water can be anywhere from 60-300 or more. As you use an RO/DI system, the TDS slowly goes up as the filter stages become exhausted or clogged. This lets you know when to replace the stages.

Undergravel Filters (UGFs)
These are another ancient filtration method you may see referenced in old antique aquarium books from the ’80s. These were slotted grates that went in the aquarium under the gravel. Riser tubes allowed the use of air bubbles or powerheads to pull water up from under the grate. The idea was that the flow of water turned the whole gravel bed into a filter media. The problem was that it greatly increased the need to vacuum the gravel, pulled gunk into places you couldn’t get to at all, and overall just increased the nitrate concentration as all that gunk rotted. In addition, sand is a far superior substrate anyway and isn’t compatible with UGFs. Don’t even think of using an UGF.

Common opening for urine, feces, and reproduction.

A special air intake method that uses the venturi effect to pull air into water. The venturi effect is the concept that water moving past a small hole will create a suction. This means that if you pump water past a small air hole it will pull air in through that hole. Some powerheads have this feature, but it is usually in reference to protein skimmers that use venturi air intakes as their method of pulling air into the skimmer.

A group of special algae that live inside corals. The corals themselves don’t use light, it is the zooxanthellae algae inside of them that do. The algae get some nutrients from the coral and use light to carry out photosynthesis. The waste from the algae is food for the corals.