Water Chemistry

Water chemistry is the part of fishkeeping that beginners underestimate and experienced aquarists obsess over. The water in your tank looks the same whether your fish are thriving or slowly dying — clear, clean, apparently fine. The truth is hidden in six or seven measurable parameters that you cannot see, smell, or guess at. You either test for them or you wait for a fish to tell you something is wrong, by which point it is often too late.

The nitrogen cycle is the foundation of all water chemistry. Fish constantly excrete ammonia through their gills and in their waste. Ammonia is highly toxic at any detectable level. A healthy filter hosts two groups of bacteria: the first converts ammonia to nitrite (also toxic), and the second converts nitrite to nitrate (much less toxic). Nitrate is removed by water changes or, in planted tanks, absorbed by plants as fertilizer. If any link in that chain breaks — a washed filter, an uncycled tank, an overstocked aquarium — ammonia or nitrite spikes and fish die within hours. The cycling guide in this section explains exactly how to build that bacterial colony before adding fish.

Beyond the nitrogen cycle, the parameters that matter are pH, GH (general hardness), KH (carbonate hardness), and temperature. pH measures how acidic or alkaline your water is on a 0 to 14 scale, with 7.0 neutral. Most community fish are happy between 6.5 and 7.5. The mistake beginners make is chasing a perfect pH instead of keeping it stable — fish tolerate a wide range of pH far better than they tolerate sudden swings. KH buffers pH against change; low KH means unstable pH. GH measures dissolved calcium and magnesium, which matter most for shrimp, snails, and livebearers.

TDS — total dissolved solids — is the parameter that separates casual aquarists from breeders. It measures everything dissolved in the water at once: minerals, organics, salts, fertilizers. Caridina shrimp breeders obsess over TDS in single-digit increments. Community keepers rarely need to test it. The water parameters guide in this section explains when each test actually matters and when you are wasting money checking things that do not affect your fish.

Three rules will save you from almost every water chemistry disaster. One: never add untreated tap water to a tank — chlorine and chloramine kill filter bacteria and burn fish gills. Always use a dechlorinator. Two: never replace all your filter media at once — you will throw away the bacteria that keep your fish alive. Rinse media in old tank water, not tap water, and replace only a portion at a time. Three: match new water temperature to tank temperature within a couple of degrees — sudden temperature shocks stress fish and trigger disease.

Several deep-dive guides on individual parameters (Understanding Ammonia, Understanding pH, Understanding the Nitrogen Cycle) are in progress and will appear as "coming soon" cards below. Until they publish, the Water Parameters Explained guide covers all six parameters in one readable overview, and the nano tank cycling guide walks through the practical step-by-step of building a bacterial colony from scratch.

How to actually use a test kit

Owning a test kit only helps if you use it correctly. Most liquid kits require specific sample sizes, a fixed number of reagent drops, and a precise shake duration — the API nitrite test, for instance, requires thirty seconds of vigorous shaking followed by a one-minute wait, and skipping either step gives you a false zero. Always rinse test tubes in distilled or RO water between tests, never in tap water (chlorine residual affects results). Test at roughly the same time of day, because pH drifts slightly through the photoperiod in planted tanks.

When you test during a cycle, the pattern you want to see is: ammonia rises, then falls to zero as nitrite rises; nitrite then falls to zero as nitrate rises. Once your tank can process a 2 ppm ammonia dose to zero ammonia and zero nitrite within twenty-four hours, with nitrate climbing, your cycle is complete and fish can go in. If nitrite stays high for weeks, your second-stage bacteria (Nitrospira) have not colonised — check pH is above 6.5, temperature is above 22°C, and add bottled bacteria such as Dr. Tim’s One and Only or Tetra SafeStart.

After your tank is cycled and stocked, a weekly testing habit catches problems before they become emergencies. Test ammonia and nitrite every week (both should always read zero), nitrate every week or two (target under 20 to 40 ppm for community tanks, lower for sensitive species), and pH monthly unless something changes. Sudden pH drops in soft-water areas signal low KH — the fix is adding crushed coral, aragonite, or a small amount of baking soda to raise buffering capacity.

A note on strips: dip-and-read strips are convenient and acceptable for routine pH, nitrate, and general hardness checks, but they are useless for ammonia — the most important parameter in a crisis. They also expire faster than liquid reagents and degrade quickly once opened. Treat strips as a quick daily sanity check and keep a liquid kit on hand for any serious troubleshooting. The cost difference is negligible over a year, and the accuracy gap is the difference between catching a problem and missing it.