Experienced aquarists have a saying: you are not keeping fish, you are keeping water. The single biggest factor in whether fish thrive or sicken is the quality of the water they live in — far more than any medication or gadget.
The key parameters
- Ammonia and nitrite: both highly toxic; a healthy, cycled tank reads zero for both.
- Nitrate: less harmful but builds up over time and is removed by water changes.
- Temperature: must suit the species and stay stable — swings stress fish.
- pH and hardness: should match what your fish need and stay consistent.
Keeping water healthy
Test the water regularly with a proper kit rather than guessing. Do routine partial water changes — never the whole tank at once — using dechlorinated water. Avoid overfeeding and overstocking, the two fastest ways to wreck water quality. Clean filter media in old tank water, never under the tap, to protect the beneficial bacteria.
When fish look unwell
At the first sign of illness — clamped fins, gasping at the surface, loss of colour or appetite — test the water before reaching for medication. More often than not, correcting a water-quality problem is the cure. Stable, clean water prevents the great majority of fish disease.