Water Testing Terms Explained

Movement of water surface or addition of air to increase dissolved oxygen and gas exchange. Critical in warm water, heavily stocked tanks, and ponds during summer.

Rapid growth of algae caused by excess nutrients, light, or both. In pools it presents as green, yellow, or black water; in aquariums and ponds it can crash oxygen levels and harm livestock.

Water's buffering capacity against pH changes. Not the same as pH itself—alkalinity measures the ability to resist pH swings. Critical for all water bodies.

Toxic nitrogen compound from fish waste, uneaten food, and decaying organic matter. The first stage of the nitrogen cycle. Even small amounts stress fish and can be fatal.

Nitrifying bacteria (Nitrosomonas, Nitrobacter) that colonize filter media and surfaces, processing ammonia and nitrite into less harmful nitrate. Essential for a healthy aquarium.

Total biological waste produced by fish, invertebrates, and decaying matter in an aquarium. Higher bioload requires more filtration capacity and more frequent water changes.

In line cartridge feeder that slowly dissolves bromine tablets into spa or hot tub water. Provides consistent sanitation between manual doses.

Alternative sanitizer to chlorine, more stable at spa temperatures (above 95°F). Produces less odor than chlorine and remains effective in a wider pH range.

Essential mineral for coral skeleton growth, invertebrate shells, and coralline algae development. Depleted by calcifying organisms and must be supplemented in reef tanks.

Dissolved calcium level that prevents water from being corrosive (low) or scaling (high). Protects pool surfaces, plumbing, and equipment from damage.

Pool or spa chemical that clumps fine particles together so they can be filtered out, restoring clarity. Different from sanitizers, which kill organisms but do not remove debris.

Essential for planted tank photosynthesis. Injected CO₂ promotes plant growth but excessive levels can stress fish by reducing available oxygen.

Chlorine bound to ammonia and nitrogen compounds, forming chloramines. Causes eye/skin irritation and the 'chlorine smell.' Removed by shocking the pool.

Protects chlorine from UV degradation in outdoor pools. Essential for sunny climates but excessive levels reduce chlorine effectiveness. Indoor pools typically don't need it.

Process of establishing beneficial bacteria colonies in a new aquarium before adding fish. Can take 4–8 weeks. Critical for fish survival and prevents 'new tank syndrome.'

Anaerobic bacterial process that converts nitrate to harmless nitrogen gas. Occurs naturally in deep sand beds, porous live rock, and specialized filter media.

German measurement unit for general hardness measuring calcium and magnesium. Common in aquarium hobby. 1 dGH ≈ 17.9 ppm CaCO₃ equivalent.

German measurement unit for carbonate hardness. Standard in aquarium hobby. 1 dKH = 17.9 ppm CaCO₃. Also called 'degrees KH' or simply 'KH.'

Complete water replacement recommended every 3–4 months for spas due to concentrated contaminants in a small volume. More frequent with heavy use or persistent water quality issues.

Reversing flow through a sand or DE pool filter to flush out trapped debris. Should be done when filter pressure rises 8 to 10 psi above clean baseline.

Foam on spa water surface caused by body oils, lotions, soaps, detergents, and high TDS. Indicates water quality issues that may require treatment or a complete drain and refill.

Active, available chlorine that sanitizes pool water by killing bacteria, algae, and pathogens. The most important sanitizer measurement for pool safety.

Measures dissolved calcium and magnesium ions in water. Affects fish osmoregulation, breeding, and shell/coral development. Different species have different GH requirements.

Common freshwater parasite causing white salt like spots on fish. Treated with elevated temperature and copper or salt based medications. Highly contagious in community tanks.

Measures carbonates and bicarbonates that buffer pH, preventing dangerous pH swings. Critical for reef tanks where stable pH is essential for coral health.

Reagent-based testing using drops and color comparison charts. More accurate and reliable than test strips for most parameters, especially ammonia and pH.

Works synergistically with calcium and alkalinity for coral health in reef tanks. Stabilizes calcium and alkalinity balance; deficiency causes calcium precipitation.

End product of the nitrogen cycle. Less toxic than ammonia/nitrite but causes fish stress, algae blooms, and coral issues when elevated. Removed through water changes and plants.

Second stage of the nitrogen cycle, produced when beneficial bacteria convert ammonia. Highly toxic to fish by binding to hemoglobin and preventing oxygen transport.

Biological process where beneficial bacteria convert toxic ammonia → nitrite → less-toxic nitrate. Foundation of aquarium health and the reason cycling is essential.

Chemical reaction that removes electrons from contaminants, neutralizing them. Chlorine, bromine, ozone, and hydrogen peroxide all sanitize through oxidation.

Supplemental sanitizer using ozone gas to oxidize contaminants. Reduces chemical demand significantly but doesn't provide residual protection—must be used with chlorine or bromine.

Logarithmic measure of hydrogen ion concentration indicating acidity (0–6.9) or alkalinity (7.1–14). 7.0 is neutral. Each whole number represents a 10x change in acidity.

Nutrient that fuels nuisance algae growth when elevated. Enters aquariums through feeding, tap water, and decaying matter. Especially problematic in reef tanks.

Granular ferric oxide media used in reef tanks and ponds to chemically bind and remove phosphate. Reduces nuisance algae fuel and helps maintain low nutrient systems.

Number of times the entire pond volume passes through filtration each hour. Koi ponds typically need a full turnover every 1 to 2 hours; ornamental ponds can run slower.

Standard unit of concentration measurement equivalent to mg/L (milligrams per liter). One ppm = 1 milligram of substance in 1 liter of water.

Saltwater filtration device that uses fine bubbles to remove dissolved organic compounds before they break down into nitrate and phosphate. Standard equipment in reef and marine systems.

Separate tank used to observe and treat new or sick fish before introducing them to a main display. Reduces disease spread and lets medications be used without affecting plants or invertebrates.

Reverse osmosis and deionized water with virtually all minerals and contaminants removed. Used as a clean base for reef tanks, pharmaceutical grade freshwater setups, and to dilute high TDS tap water.

Concentration of dissolved salts in water, critical for marine life osmoregulation. Measured in specific gravity (SG) or parts per thousand (ppt). Stability is more important than exact values.

Sodium chloride level for saltwater chlorine generators. The salt cell converts salt to chlorine through electrolysis, providing continuous sanitization.

Pool device that converts dissolved salt into chlorine through electrolysis. Produces a softer feeling pool while still sanitizing, but raises pH and demands stable salt and CYA levels.

Chemical that kills bacteria, viruses, algae, and other pathogens in water. Common sanitizers include chlorine, bromine, and ozone. Essential for safe water.

Langelier Saturation Index calculates if water is corrosive, balanced, or scale-forming based on pH, temperature, alkalinity, calcium, and TDS. Used to prevent equipment damage.

Adding high dose of chlorine (or non-chlorine oxidizer) to destroy chloramines, organic contaminants, and algae. Perform weekly or after heavy use, rain, or visible problems.

Ratio of saltwater density to pure water density. Standard method for measuring salinity in marine aquariums using a hydrometer or refractometer.

Material covering the bottom of an aquarium or pond, such as gravel, sand, or aquasoil. Affects pH, hardness, plant root health, and beneficial bacteria colonization.

Secondary tank plumbed below or beside the display tank that hides equipment, increases total water volume, and houses filtration, heaters, and skimmers.

Total Dissolved Solids measures all dissolved substances including minerals, salts, and organics. High TDS reduces sanitizer effectiveness and causes cloudy water.

Water temperature affects organism metabolism, oxygen solubility, chemical reaction rates, and sanitizer effectiveness. Stable temperatures reduce stress on aquatic life.

Paper strips with reagent pads that change color to indicate parameter levels. Fast and convenient but generally less accurate than liquid test kits.

Water's ability to resist pH changes by buffering acids. Prevents 'pH bounce' that stresses swimmers and damages equipment. Adjust before correcting pH.

Combined amount of free chlorine plus combined chlorine. Should be close to free chlorine level; a large gap indicates chloramines need to be removed by shocking.

Removing and replacing a portion of water to dilute pollutants, remove nitrates, and replenish minerals. Frequency depends on bioload, test results, and water body type.