Water chemistry is the most confusing aspect of fishkeeping for beginners. pH, GH, KH, ammonia, nitrite, nitrate — what do these mean and which ones actually matter? Here's the plain-English explanation every fishkeeper needs.

Ammonia (NH3/NH4+): the most dangerous

Ammonia is fish waste. It's toxic at any level above 0ppm. New tanks have ammonia spikes before the nitrogen cycle establishes. Symptoms: fish at the surface gasping, red gill tissue, lethargy. Fix: water changes to dilute, Seachem Prime to detoxify temporarily. Long-term: a properly cycled tank processes ammonia as fast as fish produce it.

Close-up of a laboratory technician using a pipette for precise measurement in a modern lab.Photo by Léster Lau on Pexels
Testing ammonia is the most important water test you'll do

Nitrite (NO2-): the cycling danger

Nitrite is produced when bacteria convert ammonia — it's the intermediate step in the nitrogen cycle. Also toxic to fish. Causes 'brown blood disease' — nitrite binds to hemoglobin and prevents oxygen transport. Appears in tanks during cycling and after large water changes or filter media changes. Fix: water changes, add Seachem Prime.

Nitrate (NO3-): the manageable one

Nitrate is the final product of the nitrogen cycle. Much less toxic than ammonia or nitrite. Fish can tolerate up to 40ppm, though under 20ppm is ideal. Removed by: weekly water changes, live plants (especially fast growers like hornwort). High nitrate causes: algae blooms, reduced immune function, long-term stress in fish.

Essential

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Tests all four critical parameters: ammonia, nitrite, nitrate, and pH. 800 tests. The kit every fishkeeper needs.

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pH: the acid-base balance

pH 7.0 is neutral. Below 7 is acidic, above is alkaline. Most tropical freshwater fish thrive between 6.5-7.5. Stability matters more than exact number — a stable pH of 7.4 is better than one that swings from 6.8 to 7.8. Large water changes can cause pH swings. Always dechlorinate water and let it reach room temperature before adding to the tank.

Detailed underwater shot of fish swimming in an aquarium tank, showcasing aquatic life.Photo by Guryan on Pexels
Stable water parameters are the foundation of fish health

GH and KH: hardness

GH (General Hardness) measures dissolved calcium and magnesium. KH (Carbonate Hardness) buffers pH and prevents swings. Shrimp need high GH. Discus need very soft water (low GH). Most community fish tolerate a wide range. If you have wild pH swings, your KH is probably too low — add crushed coral to raise it.

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