Koi Pond Glossary

61 terms covering water chemistry, fish health, filtration, construction, feeding, and koi variety classification. Each definition links to the relevant in-depth article.

Aerobic: Requiring oxygen. Nitrifying bacteria are aerobic organisms that need dissolved oxygen to oxidize ammonia and nitrite. Read more
Ammonia (NH₃): The toxic, un-ionized form of ammonia excreted by fish. Exists in pH- and temperature-dependent equilibrium with ammonium (NH₄⁺). Free ammonia above 0.02 mg/L causes chronic stress in koi. Read more
Ammonium (NH₄⁺): The ionized, less toxic form of ammonia. Predominates at lower pH. Does not cross gill membranes as readily as un-ionized NH₃. Read more
Anaerobic: Without oxygen. Anaerobic zones in ponds and filters support denitrifying bacteria that convert nitrate to nitrogen gas. Read more
AOB (Ammonia-Oxidizing Bacteria): Bacteria, primarily Nitrosomonas, that oxidize ammonia to nitrite in the first step of nitrification. Read more
Biofilm: A structured community of microorganisms attached to a surface and enclosed in a self-produced matrix of extracellular polymeric substances. Nitrifying bacteria form biofilms on filter media. Read more
Biological Filtration: Filtration that relies on living microorganisms (primarily nitrifying bacteria) to convert toxic ammonia and nitrite to less toxic nitrate. Read more
Bog Filter: A planted filtration system where water flows through gravel beds planted with emergent vegetation. Plants and associated microbes remove nutrients from pond water. Read more
Bottom Drain: A gravity-fed drain installed at the lowest point of a koi pond floor. Draws waste-laden water from the bottom for filtration. Read more
Brown Blood Disease: Methemoglobinemia caused by nitrite poisoning. Nitrite converts hemoglobin to methemoglobin, which cannot carry oxygen, turning blood chocolate brown. Read more
Carbonate Hardness (KH): A measure of the water's buffering capacity against pH changes, expressed in mg/L CaCO₃ or degrees KH (dKH). Maintain above 80 mg/L (4.5 dKH) in koi ponds. Read more
CCC (Criterion Continuous Concentration): EPA chronic ammonia threshold. The highest concentration of total ammonia nitrogen that aquatic organisms can be continuously exposed to without unacceptable chronic effects. Read more
Chemical Filtration: Filtration that removes dissolved substances through chemical reactions or adsorption. Includes activated carbon, zeolite, and phosphate binders. Read more
Chloramine: A disinfectant (NH₂Cl) used in municipal water treatment. More stable than chlorine and requires specific dechlorination treatment. Toxic to fish and nitrifying bacteria. Read more
CMC (Criterion Maximum Concentration): EPA acute ammonia threshold. The highest concentration of total ammonia nitrogen that aquatic organisms can be exposed to briefly without unacceptable acute effects. Read more
Columnaris: Bacterial infection caused by Flavobacterium columnare. Appears as white or grayish patches on skin, fins, and gills. Often triggered by stress and poor water quality. Read more
Cycling: The process of establishing a functioning nitrogen cycle in a new pond or filter system. Typically takes 4–8 weeks. Can be done with or without fish present. Read more
Dechlorination: The process of neutralizing chlorine or chloramine in municipal tap water before adding it to a pond. Essential during water changes. Read more
Denitrification: The anaerobic bacterial conversion of nitrate (NO₃⁻) to nitrogen gas (N₂). Occurs in oxygen-depleted zones and is the only biological pathway for removing nitrate from water. Read more
Dissolved Oxygen (DO): The amount of gaseous oxygen dissolved in water, measured in mg/L or ppm. Koi require minimum 6 mg/L; 8+ mg/L is preferred. Critical for fish respiration and nitrifying bacteria. Read more
Dropsy: A symptom complex characterized by fluid accumulation in the body cavity, causing scales to protrude ('pine cone' appearance). Often indicates severe internal bacterial infection or organ failure. Read more
Drum Filter: A mechanical filter using a rotating screen drum to remove suspended solids from pond water. Self-cleaning and efficient for high-volume koi systems. Read more
EPDM (Ethylene Propylene Diene Monomer): A synthetic rubber pond liner material. Flexible, UV-resistant, and available in large seamless sheets. Commonly used for koi pond construction. Read more
Fin Rot: Bacterial degradation of fin tissue, typically caused by Aeromonas or Pseudomonas species. Usually secondary to poor water quality or physical damage. Read more
Fluke: Parasitic flatworms (trematodes) in the genera Dactylogyrus (gill flukes) and Gyrodactylus (body flukes) that attach to fish tissue. Diagnosed by skin/gill scrape under microscope. Read more
Free Ammonia: The un-ionized form of ammonia (NH₃) that is toxic to fish. Its concentration depends on total ammonia nitrogen, pH, and temperature. Read more · Read more
GH (General Hardness): A measure of dissolved calcium and magnesium ions in water. Important for fish osmoregulation. Koi prefer GH of 100–200 mg/L. Read more
GPH (Gallons Per Hour): Unit of measurement for pump flow rate. Koi ponds typically require the full pond volume to circulate through filtration at least once every 1–2 hours. Read more
Head Pressure (Head Height): The vertical distance water must be pumped, plus friction losses from pipe length, fittings, and bends. Reduces effective pump flow rate. Measured in feet. Read more
Heterotrophic Bacteria: Bacteria that obtain energy by consuming organic compounds. Responsible for decomposing organic waste but do not perform nitrification. Can outcompete nitrifiers for oxygen and space. Read more
Ich (Ichthyophthirius multifiliis): A ciliate protozoan parasite causing 'white spot disease.' Appears as small white dots on skin and fins. Life cycle is temperature-dependent; treatable only in the free-swimming theront stage. Read more
KHV (Koi Herpesvirus): Cyprinid herpesvirus 3 (CyHV-3). A highly contagious and lethal virus specific to common carp and koi. Causes mortality rates of 80–100% in outbreaks. Active at water temperatures between 64–81°F (18–27°C). No cure exists. Read more
Kohaku: A koi variety featuring a white (shiroji) base with red (hi) markings. One of the 'Big Three' (gosanke) varieties and the most popular and historically significant koi type. Read more
Mechanical Filtration: Physical removal of suspended solids and debris from water using screens, brushes, mats, or settling chambers. Precedes biological filtration in most koi systems. Read more
Methemoglobinemia: A condition where hemoglobin is oxidized to methemoglobin and can no longer transport oxygen. Caused by nitrite absorption through the gills. Also called 'brown blood disease.' Read more
Moving Bed Biofilter (MBBR): A biological filter using small plastic media elements (carriers) that are kept in suspension by aeration. Provides high surface area for bacterial colonization with self-cleaning action. Read more
New Pond Syndrome: The period of elevated ammonia and nitrite in a new pond before the nitrogen cycle is fully established. Typically lasts 4–8 weeks without bacterial supplementation. Read more
Nitrate (NO₃⁻): The end product of nitrification. Relatively non-toxic below 30 mg/L but accumulates over time. Removed by water changes, denitrification, or plant uptake. Read more
Nitrification: The two-step aerobic bacterial process that converts ammonia (NH₃) to nitrite (NO₂⁻) and then to nitrate (NO₃⁻). The core of biological filtration in koi ponds. Read more
Nitrite (NO₂⁻): The intermediate product of nitrification. Toxic to fish — causes methemoglobinemia by oxidizing hemoglobin. Dangerous above 0.25 mg/L in koi ponds. Salt (NaCl) is the primary treatment. Read more
Nitrobacter: A genus of nitrite-oxidizing bacteria traditionally considered the primary performer of the second nitrification step. Recent research shows Nitrospira is often more important in freshwater systems. Read more
Nitrosomonas: The primary genus of ammonia-oxidizing bacteria responsible for converting ammonia to nitrite in the first step of nitrification. Slow-growing with a doubling time of 15–24 hours. Read more
Nitrospira: A genus of nitrite-oxidizing bacteria now recognized as often the dominant nitrite oxidizer in freshwater biofilters, displacing the traditionally cited Nitrobacter. Read more
NOB (Nitrite-Oxidizing Bacteria): Bacteria, including Nitrobacter and Nitrospira, that oxidize nitrite to nitrate in the second step of nitrification. Read more
Osmoregulation: The physiological process by which fish maintain proper internal salt and water balance relative to their environment. Freshwater fish continuously absorb water and excrete dilute urine. Read more
pH: The negative logarithm of hydrogen ion concentration, measuring how acidic or alkaline water is on a scale of 0–14. Koi thrive at pH 7.0–8.4. Directly affects ammonia toxicity. Read more
Phosphate (PO₄³⁻): An inorganic nutrient that drives algae growth. Primary source in koi ponds is fish food and waste. Controllable with phosphate binders, water changes, and reduced feeding. Read more
pKa: The acid dissociation constant. In ammonia chemistry, the pKa determines the ratio of NH₃ to NH₄⁺ at a given temperature. Calculated as pKa = 0.09018 + 2729.92/(T+273.15). Read more
Pond Turnover: A rapid mixing event where stratified water layers in a pond suddenly invert, bringing anoxic bottom water to the surface. Can cause acute dissolved oxygen crashes and fish kills. Read more
Quarantine: The practice of isolating new or sick fish in a separate system for observation and treatment before introducing them to the main pond. Standard duration is 4–6 weeks. Read more
RPE (Reinforced Polyethylene): A woven, reinforced polyethylene pond liner. Lighter and more puncture-resistant than EPDM but less flexible. Used in larger pond installations. Read more
Sanke (Taisho Sanshoku): A koi variety with a white base, red markings, and black (sumi) spots. One of the gosanke (Big Three) varieties alongside Kohaku and Showa. Read more
Showa (Showa Sanshoku): A koi variety with a black base and red and white markings. The third member of the gosanke (Big Three). Distinguished from Sanke by having black wrapping below the lateral line. Read more
Skimmer: A device that draws water from the pond surface, removing floating debris, pollen, and organic matter before it sinks. Essential for surface clarity. Read more
Specific Surface Area (SSA): The surface area per unit volume of filter media, measured in m²/m³. Higher SSA provides more colonization area for nitrifying bacteria. Critical metric for biomedia selection. Read more
TAN (Total Ammonia Nitrogen): The combined concentration of un-ionized ammonia (NH₃) and ionized ammonium (NH₄⁺). What standard test kits measure. Must be interpreted in context of pH and temperature. Read more · Read more
Ulcer: An open wound on the fish's body, typically caused by Aeromonas or Pseudomonas bacterial infection. Often secondary to physical damage, parasites, or immunosuppression from poor water quality. Read more
UV Clarifier: An ultraviolet light unit through which pond water is pumped. UV-C radiation (254 nm) damages the DNA of single-celled organisms, killing suspended algae and waterborne pathogens. Read more
Venturi: A device that uses the Venturi effect to inject air into a water stream. Creates fine bubbles for aeration without a separate air pump. Read more
Wheat Germ Diet: A lower-protein, easily digestible koi food used during cooler temperatures (50–65°F / 10–18°C) when fish metabolism is reduced and high-protein food cannot be fully digested. Read more
ZNA (Zen Nippon Airinkai): The All Japan Koi Lovers Association. The premier international koi judging and appreciation organization, headquartered in Japan. Sets standards for koi variety classification. Read more