Aquarium Water Parameters: The Complete Reference Guide

What is pH?

pH measures how acidic or alkaline your water is on a scale from 0 (most acidic) to 14 (most alkaline), with 7 being neutral. Most community fish tolerate a pH of 6.5–7.5 without issues.

pH matters because it affects how toxic ammonia is. At higher pH, more ammonia exists in its toxic form (NH₃). At pH 7, only ~0.5% of total ammonia is toxic; at pH 8, that rises to ~5%. This is why high pH + any ammonia is especially dangerous.

What changes pH: decaying organic matter and CO₂ lower pH over time; crushed coral, limestone rocks, and shells raise it. Tap water pH can change seasonally.

How to adjust: For soft acidic water, add driftwood or use RO/DI water. For alkaline water, use crushed coral substrate or aragonite. Avoid pH-Up/Down chemical additives — they cause unstable swings that stress fish more than a consistent "wrong" pH would.

The most urgent parameter to get right

Ammonia is produced by fish waste, uneaten food, and decaying plant matter. It's directly toxic to fish — it damages gills, burns internal tissues, and suppresses the immune system. In an established, cycled aquarium, ammonia should always read 0 ppm.

What causes ammonia spikes: new tank (not yet cycled), overfeeding, dead fish left in tank, overstocking, filter media cleaned with tap water (kills beneficial bacteria), or a sudden temperature drop.

Emergency fix: 25–30% water change immediately, then repeat daily until readings are 0. Dose Seachem Prime — it detoxifies ammonia for 24–48 hours without removing it, buying time for beneficial bacteria to process it. Do not add more fish until the tank is fully cycled.

Toxic — almost as dangerous as ammonia

Nitrite is the intermediate stage in the nitrogen cycle — Nitrosomonas bacteria convert ammonia into nitrite, and then Nitrospira bacteria convert nitrite into nitrate. Nitrite blocks red blood cells from carrying oxygen, effectively suffocating fish even in well-oxygenated water.

In an established, fully cycled tank, nitrite also reads 0 ppm at all times. If you detect nitrite in an established tank, your biological filter has been damaged — check for dead fish, disrupted filter media, or medication that kills bacteria.

Emergency fix: Water changes + Prime (as with ammonia). Adding salt (sodium chloride) at 1–3 g/L can temporarily reduce nitrite toxicity by helping fish absorb more chloride — but this is a short-term measure only.

The end product — manageable but still dangerous at high levels

Nitrate is the end product of the nitrogen cycle — far less toxic than ammonia or nitrite, but it accumulates steadily in a closed aquarium. High chronic nitrate levels suppress the immune system, slow growth, and reduce lifespan even when fish appear healthy.

Reef tanks and sensitive species like discus, cardinal tetras, and dwarf cichlids need nitrate below 5–10 ppm. Hardy fish like goldfish and many livebearers tolerate up to 40 ppm.

How to lower nitrate: Regular water changes (the most effective method), live plants (especially fast-growing stem plants), and reducing feeding/stocking density. Nitrate-absorbing resins work but get expensive long-term.

Stability matters as much as the value

Fish are ectothermic — their body temperature matches their environment. Temperature controls their metabolism, immune function, and oxygen requirements. Most tropical community fish thrive at a stable 25–26 °C.

What kills fish is not just the wrong temperature — it's rapid swings. A 3 °C (5 °F) change within a few hours can cause shock, suppressed immunity, and ich outbreaks. Always acclimate new fish slowly (30–60 min float in sealed bag, then gradual water mixing).

Practical tips: Use a quality heater rated for your tank size with 10–20% headroom. Set it 1–2 °C below your target and verify with a separate thermometer — heater thermostats can drift. In summer, consider a fan across the water surface to cool via evaporation.

Affects fish osmoregulation and mineral balance

GH measures the total concentration of calcium (Ca²⁺) and magnesium (Mg²⁺) ions in the water. These minerals are essential for fish muscle function, bone development, and egg fertilisation. Too low, and fish struggle to maintain proper fluid balance; too high, and kidneys are stressed.

Soft water fish: discus, cardinal tetras, apistogramma, most corydoras. Hard water fish: African cichlids, mollies, guppies, goldfish. Check your species requirements before adjusting.

Raising GH: crushed coral, limestone rocks, or commercial GH+ additives. Lowering GH: blend with RO (reverse osmosis) water or rainwater (use dedicated collection, not near treated roofs).

Protects your pH from crashing

KH measures bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, which act as a chemical buffer — they absorb acid and prevent pH from dropping. A tank with very low KH can experience a sudden "pH crash," dropping from 7.5 to 5.5 overnight and killing fish.

In planted tanks injecting CO₂, you intentionally lower pH through carbonic acid. Running KH too high in this setup will resist CO₂ injection. Most planted CO₂ tank keepers target 3–5 dKH with pH 6.4–6.8 during the day.

Raising KH: baking soda (NaHCO₃) — 1 teaspoon raises 100 L by about 4 dKH. Add slowly dissolved in water. Crushed coral also raises both KH and GH. Lowering KH: dilute with RO water or peat-filtered water.

Often overlooked, always essential

Dissolved oxygen is what fish extract through their gills. Unlike CO₂, oxygen enters the water at the surface — surface agitation from filters, airtstones, or powerheads is how you maintain high DO levels.

Warm water holds less oxygen than cold water — another reason summer is risky. Overstocked tanks and tanks with heavy algae blooms (which consume oxygen at night) are prone to DO crashes. Signs of low DO: fish gasping at the surface, especially in the early morning before lights come on.

How to raise DO: increase surface agitation, add an airstone, reduce temperature if possible, do a water change. In a crisis, pour a bucket of water from height into the tank to splash air into the water.

Always zero — use a dechlorinator before every water change

Tap water contains chlorine (or chloramine — a more stable chlorine/ammonia compound) to make it safe to drink. Both are toxic to fish gills and will kill beneficial bacteria in your filter.

Chlorine dissipates if you leave tap water out for 24+ hours, but chloramine does not — it's used by most modern municipal water systems. Always use a dechlorinator like Seachem Prime, API Stress Coat, or similar before adding tap water to your tank.

Prime is especially useful because it also detoxifies ammonia and nitrite for 24–48 hours — making it the go-to conditioner for both routine water changes and emergency situations.