Water chemistry is the part of fishkeeping that most beginners underestimate. A tank can look clean and the fish can appear active right up until a crash — usually caused by undetected ammonia or a pH shift that accumulated over several weeks. Understanding what the test results actually mean, and why certain parameters matter, makes it possible to intervene before problems become irreversible.
pH: what it measures and why it matters
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 to 14. A neutral reading is 7.0. Values below 7.0 are acidic; above 7.0 are alkaline. The logarithmic scale means that a pH of 6.0 is ten times more acidic than pH 7.0, and a pH of 5.0 is one hundred times more acidic — a distinction that matters when interpreting test results from week to week.
Most popular freshwater species tolerate a pH range of 6.5–7.5 without difficulty. Species from South American river systems — many tetras, discus, angelfish, and corydoras — originate in soft, acidic water (pH 5.5–6.8). African rift lake cichlids require alkaline conditions (pH 7.8–8.5). Keeping species in water significantly outside their native pH range causes chronic stress, suppresses immune function, and shortens lifespan even if the fish survive short-term.
pH and ammonia toxicity
pH interacts with ammonia in a way that is directly relevant to fish safety. Ammonia in water exists as two forms: ionised ammonium (NH₄⁺) and un-ionised ammonia (NH₃). Un-ionised ammonia is toxic to fish; ammonium is not. At higher pH values, a greater proportion of total ammonia is in the toxic un-ionised form. At pH 8.0, a total ammonia reading of 0.25 mg/L contains significantly more toxic NH₃ than the same reading at pH 7.0. For this reason, hobbyists keeping alkaline tanks need to maintain lower overall ammonia levels.
Hardness: GH and KH
Water hardness is reported in two separate measurements that are often confused:
- GH (general hardness): The total concentration of dissolved magnesium and calcium ions. Measured in °dH or ppm. Soft water has GH below 8 °dH; hard water is above 15 °dH. GH affects fish physiology — many processes, including egg fertilisation and osmoregulation, require appropriate mineral concentrations.
- KH (carbonate hardness or alkalinity): The concentration of carbonate and bicarbonate ions. KH acts as a pH buffer — the higher the KH, the more resistant the water is to pH swings. Low KH water (below 3 °dH) is prone to pH crashes, especially in planted tanks where CO2 injection lowers pH.
Warsaw tap water is moderately hard, typically around 10–14 °dH GH and 6–10 °dH KH. This suits a wide range of community fish but requires adjustment (via RO water dilution or blackwater extract) for softwater species like discus or wild-type cardinal tetras.
The nitrogen compounds: ammonia, nitrite, nitrate
These three parameters are central to understanding the nitrogen cycle and the biological health of the tank.
Ammonia (NH₃/NH₄⁺)
Ammonia is the primary waste product from fish metabolism and the decomposition of uneaten food. In a cycled tank, ammonia should read zero at all times. Even a reading of 0.25 mg/L (ppm) indicates a problem — filter disruption, overfeeding, a dead fish, or the tank having not completed its cycle. Exposure to elevated ammonia damages gill tissue, making it harder for fish to extract oxygen even after the ammonia is removed.
Nitrite (NO₂⁻)
Nitrobacter and related bacteria convert ammonia to nitrite. Nitrite is also highly toxic — it interferes with haemoglobin's ability to carry oxygen, causing fish to gasp at the surface even in well-oxygenated water. Like ammonia, nitrite in a cycled tank should read zero. Elevated nitrite often occurs in newly established tanks or after a large water change that disrupts the filter bacteria.
Nitrate (NO₃⁻)
The end product of the nitrogen cycle in a standard aquarium. Nitrate is relatively non-toxic at low concentrations but causes chronic stress and increased disease susceptibility at levels above 40 mg/L over extended periods. Regular partial water changes (20–30% weekly) export nitrate. Heavily planted tanks consume nitrate as a fertiliser and may keep levels near zero without water changes, though this approach requires careful monitoring.
The most common water testing schedule for an established community tank: test ammonia and nitrite weekly for the first three months; then test nitrate fortnightly and pH monthly unless symptoms appear. During cycling, test ammonia and nitrite daily.
Dissolved oxygen
Oxygen enters the water primarily at the surface through gas exchange. Agitation — from filter return flow, air stones, or surface movement — increases the rate of this exchange. At 25°C, water holds approximately 8.3 mg/L of dissolved oxygen at saturation. Most freshwater fish require at least 5–6 mg/L. Signs of low oxygen include fish gasping near the surface, particularly in the morning when plant photosynthesis has stopped overnight and biological processes have been consuming oxygen.
Temperature and its effect on chemistry
Temperature affects almost every chemical process in the aquarium. Warmer water holds less dissolved oxygen and accelerates bacterial and chemical reactions — including the conversion of ammonia to its more toxic un-ionised form. For tropical community tanks, keeping temperature stable between 24°C and 27°C is a reasonable target. Large, sudden temperature swings (more than 2°C in a few hours) stress fish and can trigger disease outbreaks.
Testing equipment and reliability
Liquid test kits are significantly more accurate than strip tests, particularly for ammonia and nitrite. The API Master Test Kit is widely available in Poland and provides reliable results for the key parameters. Strip tests oxidise quickly once opened and give imprecise readings that can be misleading — a strip showing "safe" levels of ammonia when the actual level is 0.5 mg/L can result in fish deaths before the owner realises a problem exists.
For KH and GH, separate test kits using colour titration are the most accurate method. Digital pH meters drift over time and require regular calibration with buffer solution to remain accurate. A liquid pH test kit is a reliable backup.
Adjusting parameters
The safest way to adjust pH or hardness is to blend tap water with RO (reverse osmosis) water in a known ratio. Chemical additives that claim to instantly adjust pH (such as sodium bicarbonate or phosphoric acid solutions) can cause rapid swings that are stressful to fish. Gradual adjustment over several days via water changes using pre-conditioned water is preferable. Driftwood and botanicals (catappa leaves, alder cones) lower pH and KH slowly and naturally — a method particularly suited to South American biotope setups.
Sources and further reading
For species-specific water parameter requirements, FishBase maintains a comprehensive database with original field data. The Seachem technical library provides detailed notes on water chemistry interactions relevant to aquarium hobbyists.