Optimal pH, temperature, dissolved oxygen, ammonia, and salinity ranges for 12 key aquaculture and aquarium species. Click a species to view its full parameter profile instantly.
Click any species to see all water quality thresholds instantly - no page reload needed.
Water is not simply a habitat for fish - it is their entire life-support system. Every physiological process a fish undertakes, from respiration to digestion to immune function, is directly influenced by the chemistry of the surrounding water. When parameters fall outside a species' tolerance range, even briefly, the consequences range from chronic stress and reduced growth to acute toxicity and mass mortality. Understanding and maintaining species-specific thresholds is therefore the single most important skill in both recreational fish keeping and commercial aquaculture.
Temperature governs metabolic rate in fish, which are ectotherms (cold-blooded). As temperature rises within the optimal range, fish metabolise faster, eat more, and grow quicker. Beyond the maximum tolerable temperature, enzyme systems begin to denature and oxygen dissolves less effectively in water - a lethal double jeopardy. Cold-water species such as salmon and rainbow trout require temperatures below 18–20°C, while warm-water species like tilapia, catfish, and discus thrive at 26–32°C. Rapid temperature swings (more than 2°C per hour) are nearly always more dangerous than a sustained slightly sub-optimal temperature, as they prevent physiological adaptation.
pH measures the concentration of hydrogen ions in water, on a logarithmic scale from 0 (acidic) to 14 (alkaline). Most freshwater fish thrive at a pH of 6.5–8.0, but species differ significantly. Discus and many South American species prefer slightly acidic, soft water (pH 5.5–6.8), while marine and brackish species often require pH 8.0–8.4. Extreme pH outside a species' tolerance denatures enzymes, disrupts osmoregulation, and causes gill and skin damage. pH also interacts critically with ammonia: at higher pH, a greater proportion of total ammonia nitrogen (TAN) exists as the toxic un-ionised form (NH₃) rather than the relatively harmless ionised ammonium (NH₄⁺).
Fish require dissolved oxygen (DO) for aerobic respiration. Cold-water species like salmon and trout have high oxygen demands, typically requiring DO above 7 mg/L, while some tropical species such as catfish and betta can tolerate surprisingly low DO (2–3 mg/L) due to auxiliary breathing organs or behavioural adaptations. In aquaculture, DO crashes - caused by algal blooms, high stocking density, or equipment failure - are among the most common causes of mass mortality. A minimum DO of 5 mg/L is a safe working threshold for most commercial species; below 3 mg/L, even tolerant species begin to show stress.
The nitrogen cycle is central to aquaculture and aquarium management. Fish excrete ammonia directly through their gills, and uneaten feed decomposes into ammonia. Beneficial bacteria (Nitrosomonas) oxidise ammonia to nitrite (NO₂⁻), and Nitrobacter further oxidise nitrite to the relatively harmless nitrate (NO₃⁻). Ammonia is the most acutely toxic compound: even 0.02 mg/L of un-ionised NH₃ can cause chronic gill damage in sensitive species like salmon. Robust species like tilapia and catfish tolerate TAN up to 0.5 mg/L but still suffer growth suppression. Nitrite binds to haemoglobin, causing methemoglobinaemia (brown blood disease), and maximum safe levels are similarly low. Nitrate is far less acutely toxic but causes chronic stress and growth reduction above 40–100 mg/L depending on species.
Salinity (measured in parts per thousand, ppt) determines the osmotic challenge fish face. Freshwater fish (FW) maintain higher internal salt concentrations than their environment and must constantly excrete water through urination. Marine fish face the opposite challenge. Euryhaline species like tilapia are remarkably adaptable and can tolerate salinity up to 15 ppt or even higher temporarily. General hardness (GH), measured in degrees of hardness (dGH), reflects calcium and magnesium content and affects osmoregulation, egg fertility, and skeletal development. Soft-water species like discus suffer in hard water, while livebearers (guppies, mollies) prefer moderately hard, alkaline conditions.
Regular water testing is non-negotiable in any fish system. Liquid test kits are more accurate than paper strips and should be the standard in serious operations. For aquaculture, multiparameter meters (measuring DO, temperature, pH, and conductivity simultaneously) are cost-effective investments. Test ammonia and nitrite weekly in new systems, and at least monthly in established ones. In recirculating aquaculture systems (RAS), automated continuous monitoring with alarms is standard practice. Always cross-reference readings: a high pH reading alongside low alkalinity, for instance, suggests a CO₂-driven fluctuation rather than true alkalinity, requiring a different response.
Water parameters do not operate in isolation. Temperature affects DO saturation, pH affects ammonia toxicity, hardness buffers pH, and salinity affects osmoregulation. A holistic approach - rather than chasing individual numbers - produces the best outcomes. In practice, this means maintaining stable conditions through regular partial water changes, adequate biofiltration, controlled feeding rates, and monitoring. Use this guide as a species-specific checklist: identify which parameters require the most attention for your chosen species, establish a testing routine, and act quickly when readings drift outside the optimal zones shown above.