Aquarium Terms Glossary

Every fishkeeping community has its own jargon — acronyms, scientific terms, slang. This is the glossary for understanding forum discussions, video tutorials, product descriptions, and articles. Alphabetical for quick lookup.

At a glance: the acronyms

Full entries for all of these, and everything else, are alphabetical below.

A

Acclimation — Slowly adjusting new fish to your tank's water (temperature, parameters) before releasing them.

ADA — Aqua Design Amano; influential Japanese aquascape company founded by Takashi Amano. Also refers to their products and styles.

Adipose fin — Small fleshy fin on the back of some fish (corydoras, tetras) between dorsal and tail.

Aeration — Adding oxygen to water; typically via airstone or surface agitation.

Airstone — Porous stone that diffuses air into fine bubbles.

Algae — Photosynthetic organisms (plants and bacteria) that grow in tanks. Common types: green, brown (diatoms), black beard, hair, GSA, GDA.

All-in-one (AIO) — Aquarium with built-in filtration in a separate back chamber.

Ammonia (NH₃) — Toxic fish waste; first stage of nitrogen cycle; should be 0 in established tanks.

Anabantoid / Anabantid — Fish with labyrinth organ allowing air-breathing (gouramis, bettas).

Anaerobic — Without oxygen; dangerous in substrate (gas buildup, toxic compounds).

Aquascape — Designed aquatic landscape; deliberately arranged hardscape + plants.

Aquarium controller — Computer system monitoring/controlling tank parameters (Apex, etc.).

Aquasoil — Specialized substrate for planted tanks (ADA, Tropica, Fluval Stratum); contains nutrients, lowers pH.

B

Beneficial bacteria — Bacteria converting ammonia → nitrite → nitrate; the heart of the nitrogen cycle.

Berried — Female shrimp carrying eggs (visible under tail).

Biotope — Recreating a specific natural environment with appropriate species, substrate, hardscape.

Bioload — Total fish/plant mass producing waste; affects filtration needs.

Biofilm — Microscopic life film on surfaces; essential for fry, shrimp; primary algae food source.

Blackwater — Tannin-stained water; mimics natural acidic environments (Amazon, Southeast Asian rivers).

Bristlenose — Type of pleco (Ancistrus) with bristles on face; smaller than common plecos.

Bubble nest — Foam structure built by bettas and gouramis for egg-laying.

C

Caridina — Genus of shrimp including crystal shrimp, bee shrimp, tiger shrimp.

Carbon (Activated carbon) — Filter media that adsorbs organics, tannins, medications.

Caretaker fish / Algae eater — Fish that consume algae (otocinclus, plecos, some catfish).

Centerpiece fish — A standout, often larger fish that's the visual focus.

Cherry shrimp — Common name for Neocaridina davidi var. red; popular beginner shrimp.

Chloramine — Combined chlorine + ammonia in tap water; harder to remove than chlorine alone.

Cleanup crew (CUC) — Algae eaters, scavengers, snails that help maintain the tank.

CO2 — Carbon dioxide; required for photosynthesis; injected in high-tech planted tanks.

Coldwater fish — Fish needing temps below 72°F (goldfish, white clouds, hillstream loaches).

Community tank — Multi-species peaceful tank.

Coral — Marine invertebrate; key feature of reef tanks.

Cory — Short for corydoras catfish.

CRS — Crystal Red Shrimp.

Cryptocoryne / Crypt — Genus of beginner-friendly aquarium plants.

Cycle / Cycling — Establishing nitrogen cycle in new tank; takes 4-6 weeks.

D

Dechlorinator — Liquid removing chlorine/chloramine from tap water.

DGH / dGH — Degrees of General Hardness (1 dGH = 17.86 ppm). Common hardness measure.

dKH — Degrees of Carbonate Hardness; buffering capacity.

DI — Deionization; produces extremely pure water.

Diatom — Brown algae often appearing in new tanks; harmless; fades.

Diffuser — Device dispersing CO2 into fine bubbles in water.

Discus — Large flat round cichlid; demanding species.

Display tank — The main viewing aquarium.

Drop checker — Device measuring CO2 levels; color changes indicate concentration.

Dropsy — Disease causing internal fluid buildup; pinecone scales.

Dry start — Method of growing carpet plants without water before flooding tank.

E

EI — Estimative Index; fertilization method dosing macros + micros 3x/week with weekly water changes.

Eheim — Long-established German aquarium equipment brand.

Endler — Endler's livebearer; small colorful livebearer related to guppies.

Epiphyte — Plant growing on hardscape (anubias, java fern); rhizome must not be buried.

F

Filter cycling — Establishing nitrogen-cycling bacteria in filter media.

Filter media — Materials inside filter (sponge, ceramic, carbon).

Fishless cycling — Cycling without fish, using ammonia.

Flashing — Fish rubbing against objects; sign of parasites or stress.

Flow — Water movement; important for some species.

Fluidized bed filter — Filter with bio media constantly tumbling.

Foreground — Front of tank; usually short plants or carpet.

Free swimming — Fry that have absorbed yolk sac and now swim/feed independently.

Fry — Baby fish.

FOWLR — Fish-Only With Live Rock (saltwater).

G

GH — General hardness; measures dissolved calcium/magnesium.

Gluteraldehyde — Active ingredient in liquid carbon (Excel); algicide.

Goldfish — Coldwater fish; common but demanding.

Gourami — Anabantoid fish; many community species (dwarf, pearl, blue).

Gravel vacuum — Siphon device for cleaning substrate during water changes.

Greenwater — Free-floating algae causing water to appear green; can be controlled with UV.

H

Hairgrass — Carpet plant (Eleocharis); blades of grass-like leaves.

Hardscape — Rocks and driftwood in an aquascape.

Hatchery (brine shrimp) — DIY setup for hatching baby brine shrimp.

HC Cuba — Hemianthus callitrichoides; carpet plant requiring CO2.

Heat sink — Aquarium location absorbing heat; sometimes used to describe heater behavior.

HOB — Hang-On-Back filter.

Hospital tank — Separate tank for treating sick fish.

I

Ich (Ick) — Ichthyophthirius multifiliis; white spot parasite.

Inert — Not releasing minerals or affecting chemistry; opposite of buffering.

Inline diffuser — CO2 diffuser placed in canister filter line.

Iwagumi — Minimalist aquascape style; 3, 5, or 7 stones with carpeting plant.

J

Java fern — Hardy beginner plant attached to hardscape.

Java moss — Hardy moss for hardscape attachment.

K

KH — Carbonate hardness; buffering capacity.

L

Labyrinth fish — Fish with labyrinth organ for air-breathing (gouramis, bettas).

LED — Light Emitting Diode; modern aquarium lighting.

Lighting period (photoperiod) — How long lights are on; typically 8-10 hours.

Liquid carbon — Glutaraldehyde-based; cheap alternative to CO2 for low-light plants.

Live food — Living food organisms (brine shrimp, daphnia, microworms).

Live rock — Marine rock colonized with beneficial organisms.

M

Macros / Macronutrients — Nitrogen, phosphorus, potassium for plants.

Mature tank — Established 6+ months; biological filtration robust.

Methylene blue — Antifungal/antiparasitic; for eggs and mild fungus.

Micros / Micronutrients — Iron, manganese, zinc, copper, boron, molybdenum.

Midground — Middle area of tank; medium-height plants.

Mini-cycle — Brief ammonia/nitrite spike from cycle disruption.

Mulm — Detritus, fish waste, decaying organic matter; not all bad — contains beneficial bacteria.

N

N — Nitrogen.

Nano — Small tank (typically under 10 gallons).

Neocaridina — Genus including cherry shrimp; hardy beginner shrimp.

Neritina — Genus of nerite snails; algae eaters.

Nitrate (NO₃) — End product of nitrogen cycle; plants use; removed by water changes.

Nitrite (NO₂) — Toxic intermediate in nitrogen cycle.

Nitrogen cycle — Bacterial conversion of ammonia → nitrite → nitrate.

O

Otocinclus / Otto — Small algae-eating catfish.

Overfeeding — Most common beginner mistake; causes ammonia, algae, fish issues.

Overflow — Pipe or chamber removing water from display tank to sump.

Oxygenation — Dissolved oxygen in water; affected by surface agitation, temperature.

P

P — Phosphorus.

PAR — Photosynthetically Active Radiation; measures usable light for plants.

Parasite — Organism living on/in host; ich, velvet, gill flukes are common aquarium parasites.

Pearling — Plants visibly releasing oxygen bubbles during strong photosynthesis.

Pectoral fin — Side fins (like arms).

pH — Acidity/alkalinity measure; scale 0-14, 7 is neutral.

Pleco — Plecostomus; armored catfish; many species.

ppm — Parts per million; common concentration measure.

ppt — Parts per thousand; common salinity measure.

Power filter — Filter with internal pump (HOB or internal).

PPS / PPS-Pro — Perpetual Preservation System; precise daily fertilization method.

Praziquantel — Antiparasitic medication for flukes, internal worms.

Protein skimmer — Saltwater equipment removing organic compounds.

Q

Quarantine — Isolating new fish in separate tank before adding to display.

QT — Quarantine tank.

R

Rack — Multiple stacked tanks (breeding setup).

Refugium — Sump chamber with macroalgae, copepods; natural filtration.

Remineralizer — Salts added to RO water to restore minerals.

Reverse osmosis (RO) — Water filtration removing dissolved minerals.

Reef tank — Marine aquarium with corals + fish + invertebrates.

Rescape — Major redesign of an aquascape.

Rhizome — Modified stem of epiphytic plants (anubias, java fern); must not be buried.

Riparium — Tank with plants growing partially above water.

RO/DI — Reverse Osmosis + Deionization; ultra-pure water.

S

Salinity — Dissolved salt content; measured in ppt or specific gravity.

Salt — Aquarium salt (NaCl); not the same as marine salt mix (contains many minerals).

Saltwater — Marine aquarium; salinity 30-35 ppt.

Schooling fish — Species requiring groups (6+) to thrive.

Sg (Specific gravity) — Density measure for saltwater (1.024-1.026 typical).

Shoal — Group of fish (loose schooling).

Slime coat — Mucus layer on fish skin; defense against pathogens.

Solenoid — Electric valve; common in CO2 systems for automated on/off.

SPS (Stony Polyp Stony) — Hard coral with small polyps (Acropora).

Stem plant — Plant grown from cut stem (most planted tank stems).

Substrate — Bottom material (sand, gravel, aquasoil).

Sump — Separate filtration tank below display.

T

Tank cycling — See nitrogen cycle.

Tannins — Compounds from wood/leaves tinting water amber.

TDS — Total Dissolved Solids; total mineral content.

Tetra — Family of small schooling community fish.

Top off — Adding fresh water to replace evaporation.

U

UV sterilizer — UV light unit killing free-floating organisms.

V

Vallisneria / Val — Grass-like aquarium plant.

Velvet — Oodinium parasitic disease; gold/brown dusting.

W

Wabi-kusa — Japanese-style emergent plant compositions.

Water column — The body of water itself, distinguishing from substrate.

Water change — Removing/replacing tank water; primary maintenance task.

Water conditioner — Dechlorinator + sometimes other additives.

X-Y-Z

Xenia — Type of soft coral; pulsing motion.

Yolk sac — Reserve fry rely on for first days post-hatching.

Zebra fish — Common name; usually refers to zebrafish (danio).

Zone — Distinct ecological area in a tank (territory, hiding spot).

Common Acronyms

ADA — Aqua Design Amano AIO — All-in-one ATO — Auto top-off BBS — Baby brine shrimp CRS — Crystal Red Shrimp EI — Estimative Index FOWLR — Fish-Only With Live Rock GH — General Hardness GFO — Granular Ferric Oxide GSA — Green Spot Algae HOB — Hang-On-Back filter KH — Carbonate Hardness LFS — Local Fish Store LPS — Large Polyp Stony coral N, P, K — Nitrogen, Phosphorus, Potassium PAR — Photosynthetically Active Radiation PPM — Parts Per Million PPT — Parts Per Thousand QT — Quarantine Tank RO — Reverse Osmosis SPS — Small Polyp Stony coral TDS — Total Dissolved Solids UV — Ultraviolet (sterilizer)

The Bottom Line

This glossary covers most fishkeeping jargon you'll encounter — acronyms (TDS, KH, GH, PAR, EI), scientific terms (epiphyte, anabantoid, periphyton), product categories (aquasoil, dechlorinator, protein skimmer), and slang (pearling, mulm, slime coat). Use it as a lookup when reading articles, forums, or product descriptions. Some terms have variations across regions and communities — context usually clarifies. Beyond this glossary, specialized subfields (shrimp keeping, biotope, planted tanks, saltwater) have their own deeper jargon — encountered as you specialize. Bookmark this page for when you encounter unfamiliar terms in your fishkeeping journey.

Related guides

• The complete fishkeeping reference library
• Water parameters cheat sheet
• Understanding pH, KH and GH
• How does the aquarium nitrogen cycle work?
• Choosing the right filter: HOB vs sponge vs canister
• Common aquarium diseases: treatment lookup
• Fish compatibility quick reference
• Aquarium plants quick reference

Got questions about your specific setup? Drop them in our Community Forum — we love helping new fishkeepers get started right.