Enter your aquarium readings and get an instant colour-coded assessment with advice for freshwater and marine tanks.
Water quality is the single most important factor in the health, longevity, and vibrancy of your aquarium fish. More fish die from poor water chemistry than from any other cause - including disease, overfeeding, or aggression. Yet many aquarists, especially beginners, skip water testing entirely or test only when fish appear sick. By then, the damage is often already done.
This guide explains every major water parameter you need to monitor in freshwater and marine aquariums: what each one means biologically, what the safe ranges are, how to test for them, and exactly what to do when readings fall outside the ideal range.
pH measures the concentration of hydrogen ions in water on a logarithmic scale from 0 (extremely acidic) to 14 (extremely alkaline), with 7 being neutral. Each pH unit represents a tenfold change in acidity - so pH 6 is ten times more acidic than pH 7, and pH 8 is ten times more alkaline.
For freshwater community fish, the target range is 6.8–7.5. Many popular species - neon tetras, angelfish, discus - originate from soft, acidic Amazonian waters (pH 5.5–6.5), while African cichlids from the Rift Valley lakes prefer alkaline conditions (pH 7.8–9.0). Marine aquariums must maintain pH between 8.1 and 8.4 to support coral calcification and fish health.
pH naturally falls in established tanks as biological activity produces CO₂ and acids. Signs of acidic drift: fish swimming sluggishly, breathing at the surface, or colour fading. To raise pH: perform partial water changes with properly buffered water, add crushed coral or limestone, increase surface agitation (which degasses CO₂). To lower pH: add driftwood, peat, or Indian almond leaves; use CO₂ injection for planted tanks.
Fish are ectotherms - their body temperature matches the water. Temperature affects metabolism, immune function, oxygen solubility, and the toxicity of ammonia. Rapid temperature swings (more than 2°C in 24 hours) are acutely stressful and can trigger ich (white spot disease) outbreaks.
Freshwater community fish: target 24–26°C. Cold-water species like goldfish prefer 10–18°C and should not be kept in tropical tanks. Marine reef tanks: 25–27°C (corals are sensitive to high temperatures - bleaching occurs above 28°C).
These three parameters are intimately linked and represent the most important chemistry to monitor in any aquarium. Together they describe the nitrogen cycle - the biological process that makes aquariums liveable for fish.
Fish excrete ammonia directly through their gills as a metabolic waste product. Food waste, dead plant matter, and uneaten food also decompose to ammonia. In established tanks, beneficial bacteria (primarily Nitrosomonas) convert ammonia to nitrite within hours. In new tanks that haven't been "cycled," ammonia builds to lethal levels rapidly - this is called "New Tank Syndrome."
The safe level is effectively zero. Even 0.25 mg/L causes gill inflammation; 1.0 mg/L can kill fish within 24 hours in acute exposure. If ammonia shows on a test kit, immediate action is required: emergency water change of 30–50%, stop feeding, and add a dechlorinator that detoxifies ammonia (like Prime/Seachem).
Nitrite is the intermediate compound in the nitrogen cycle. Nitrosomonas bacteria produce it from ammonia; Nitrobacter bacteria consume it. Nitrite blocks haemoglobin from binding oxygen, causing a condition called methemoglobinemia - fish literally suffocate despite adequate dissolved oxygen. Safe level: zero. Any reading indicates a cycle problem: too little bacterial colony for the bioload, filter disruption (cleaning filter media in tap water kills bacteria), or sudden stocking increase.
Nitrate is the end product of the nitrogen cycle. It's far less toxic than ammonia or nitrite and accumulates slowly. Most freshwater fish tolerate up to 40 mg/L without obvious distress, but chronic high nitrate suppresses immune function and colours fish poorly. Nitrate is removed by regular partial water changes (25–30% of tank volume weekly), plant uptake in heavily planted tanks, and algae scrubbers in some marine setups. Marine reef tanks require nitrate below 5 mg/L to maintain coral health.
Hardness describes dissolved mineral content and is particularly important for freshwater tanks.
GH measures dissolved calcium and magnesium ions. Measured in degrees of general hardness (dGH) or parts per million (ppm; 1 dGH ≈ 17.9 ppm). Very soft water (<4 dGH) can stress species adapted to harder conditions; very hard water (>20 dGH) can inhibit egg fertilisation in soft-water species. For community tanks: 5–15 dGH suits most species. African cichlids prefer >15 dGH. Discus and South American species prefer <6 dGH.
KH measures dissolved carbonate and bicarbonate - the substances that resist pH change (buffer capacity). KH is perhaps the most underappreciated parameter in aquarium keeping. A tank with low KH (<3 dKH) will experience dangerous pH crashes - sudden drops in pH when acids accumulate faster than the buffer can neutralise them. Fish can go from apparently healthy to dead overnight in a pH crash. Target KH: 4–8 dKH for freshwater; 8–12 dKH for marine.
Natural ocean water has a specific gravity (SG) of approximately 1.025–1.026 and a salinity of 35 ppt. Reef aquariums target the same range. Lower salinity (1.020–1.022) reduces the risk of some marine parasites and is used in fish-only systems, but stresses corals and invertebrates. Measure with a refractometer (more accurate than swing-arm hydrometers).
Marine alkalinity (measured as dKH or meq/L) is functionally similar to freshwater KH but at a higher target range. It buffers the marine tank's pH and is consumed by corals and other calcifying organisms as they build skeletons. Reef tanks require regular alkalinity supplementation (two-part dosing, calcium reactor, or kalkwasser). Target: 8–12 dKH.
Corals, coralline algae, clams, and other calcifying organisms incorporate calcium into their skeletons. Calcium is rapidly depleted in mature reef tanks. Target: 380–450 mg/L. Calcium and alkalinity must be dosed in tandem - raising one often depletes the other.
Liquid test kits (API, Salifert, Hanna) are more accurate than strip tests. For critical parameters (alkalinity, calcium, phosphate), titration kits or electronic meters are preferred.