Water science
Most aquarium failures are water failures, and most water advice in circulation is a simplification of a simplification. These pages keep the qualifications that usually get trimmed off.
Parameter (6)
- pHpH measures the activity of hydrogen ions on a logarithmic scale. In freshwater aquariums it matters mostly because it decides how toxic ammonia is, and because most fish tolerate a stable wrong pH better than an unstable right one.
- General hardness (GH)General hardness measures dissolved calcium and magnesium. It matters biologically — fish and invertebrates use those ions for osmoregulation, and shrimp and snails need calcium to build shell and exoskeleton.
- Carbonate hardness (KH)KH measures carbonate and bicarbonate, which neutralise acid. It is the tank's resistance to pH change — low KH means pH moves easily, high KH means it barely moves at all.
- Dissolved oxygenOxygen enters the water at the surface and is consumed by fish, plants at night, and above all by the bacteria in the filter and substrate. Warm water holds less of it, which is why heat and oxygen problems arrive together.
- TemperatureTemperature sets the metabolic rate of everything in the tank — fish, plants, bacteria and pathogens alike — and determines how much oxygen the water can hold.
- Water flowFlow moves oxygen and nutrients through the tank, keeps waste in suspension where the filter can collect it, and is itself a habitat requirement — hillstream species need it and blackwater species need the absence of it.
Compound (7)
- AmmoniaAmmonia is the primary nitrogenous waste of fish and the most acutely toxic compound in a freshwater aquarium. How dangerous a given test reading is depends on pH and temperature, because only the un-ionised NH₃ fraction is highly toxic.NH₃
- AmmoniumAmmonium is the charged form of ammonia that dominates in acidic water. It is far less toxic to fish because the charge stops it crossing the gill membrane easily, and it is the form plants take up preferentially.NH₄⁺
- NitriteNitrite is the intermediate product of the nitrogen cycle. It is taken up across the gills and oxidises haemoglobin to methaemoglobin, which cannot carry oxygen — so a fish in nitrite suffocates in fully oxygenated water.NO₂⁻
- NitrateNitrate is the end product of the aquarium nitrogen cycle and is far less toxic than ammonia or nitrite. It accumulates continuously and is removed by water changes and plant growth.NO₃⁻
- Chlorine and chloramineBoth are used to disinfect mains water and both are toxic to fish and to filter bacteria. Chlorine gasses off if water is left standing; chloramine does not, and must be neutralised chemically.
- Carbon dioxide in the aquariumCO₂ is the carbon source for plant growth and is normally the limiting factor in a brightly lit planted tank. It also forms carbonic acid, so injecting it lowers pH.CO₂
- PhosphatePhosphate is a plant nutrient that accumulates from food and waste. In planted tanks it is dosed deliberately; in unplanted tanks it is often blamed for algae with less justification than it deserves.PO₄³⁻
Process (6)
- The nitrogen cycleBacteria and archaea in the filter and on every surface convert ammonia from fish waste into nitrite, and nitrite into far less toxic nitrate, which is removed by water changes and plants.
- Biological filtrationBiological filtration is the microbial oxidation of ammonia and nitrite on surfaces inside the filter. It is what makes a closed aquarium possible.
- Mechanical filtrationMechanical filtration traps suspended particles so they can be removed from the system. It does not remove anything dissolved.
- Chemical filtrationChemical filtration uses adsorbent or ion-exchange media to remove specific dissolved substances — tannins, medications, phosphate, organics.
- Beneficial bacteriaThe microbial community that performs nitrification — mostly ammonia-oxidising bacteria and archaea plus nitrite-oxidising Nitrospira — living as biofilm on filter media and every other surface.
- Algae in the aquariumAlgae grow where light, nutrients and carbon are out of balance. Which type appears is a useful diagnostic, because different algae are associated with different imbalances.
Equilibrium (1)
Practice (3)
- Water changesReplacing part of the tank water dilutes nitrate and dissolved organics, replaces consumed minerals and restores carbonate buffer. It is the single most effective maintenance action available.
- Fishless cyclingEstablishing the filter's bacterial population by dosing pure ammonia to a target concentration and waiting until the tank converts it to nitrate within 24 hours, before any fish are added.
- Source waterEverything in the tank starts as whatever comes out of the tap. Knowing your supply's hardness, pH, nitrate and disinfectant is the starting point for deciding what you can keep easily.
Measurement (2)
- Water testingTesting tells you what is happening in the water before the fish do. The essential set for a freshwater tank is ammonia, nitrite, nitrate, pH, KH and GH.
- TDS and conductivityA TDS meter measures electrical conductivity and multiplies it by a conversion factor. It gives a single number for total dissolved ionic content, and says nothing about which ions those are.