17 · Water Chemistry & Testing
You cannot manage what you cannot measure. A basic test kit turns guesswork into a clear picture of your pond's living balance.
Parameters that matter
| Parameter | Healthy pond range | Why it matters |
|---|---|---|
| pH | 6.5 – 8.5 (stable > exact) | Extremes stress fish and stall bacteria |
| KH (carbonate hardness) | 80 – 200 mg/L (4–11°dKH) | Buffers pH; prevents dangerous crashes |
| GH (general hardness) | 100 – 250 mg/L | Calcium/magnesium for fish & snails |
| Ammonia (NH₃) | 0 mg/L | Toxic; from waste and decay |
| Nitrite (NO₂⁻) | 0 mg/L | Toxic intermediate |
| Nitrate (NO₃⁻) | < 40 mg/L ideal, < 80 acceptable | Plant food; high = algae fuel |
| Dissolved oxygen | > 5 mg/L (warm months) | Needed by fish and bacteria |
| Temperature | Depends on climate | Drives metabolism and O₂ capacity |
The most important: KH and pH stability
KH is your pH shock absorber. As bacteria convert ammonia to nitrate they release hydrogen ions, which push pH down. KH (dissolved carbonates) neutralizes those ions. When KH is depleted, pH can "crash" overnight and kill fish and filter bacteria at once.
Watch out
If your pH swings more than ~1.0 between evening and morning, your KH is likely too low. Raise it with a pond buffer or a small amount of bicarbonate of soda (test and dose carefully — ~1 teaspoon per 1000 L raises KH modestly; retest).
How to test
- Drop-test kits (liquid): Most accurate for ammonia/nitrite/nitrate/pH/KH/GH. Slightly fiddly but reliable.
- Test strips: Convenient and fine for trends, but less precise and shorter shelf life.
- Digital meters: Great for pH and TDS; need calibration and care.
- ORP / probe systems: Advanced; measure oxidation-reduction potential (water's "cleanliness" / disinfection capacity).
Testing rhythm
- New pond (first 6–8 weeks): test ammonia, nitrite, nitrate every 2–3 days — this is the "cycling" period.
- Established pond: a full test monthly, plus pH/KH if you see a problem.
- After shocks: heavy rain, adding fish, medication, or a big algae die-off — test immediately.
Interpretation shortcut
Ammonia or nitrite above zero in a mature pond means something died, you overfed, or the filter stalled. Nitrate creeping past 80 mg/L means too many nutrients — add plants or do a partial water change.
Testing procedure
- Collect a sample from 300 mm below the surface, mid-pond.
- Use the test kit within 15 minutes of collection (values shift).
- Record results in a notebook or spreadsheet with the date.
- Test at the same time each week for consistent comparison.
- Test after any major event: heavy rain, new fish, plant die-off.
Key parameters at a glance
| Parameter | Ideal range | Danger zone |
|---|---|---|
| Ammonia (NH₃) | 0 mg/L | >0.5 mg/L |
| Nitrite (NO₂⁻) | 0 mg/L | >0.5 mg/L |
| Nitrate (NO₃⁻) | <50 mg/L | >100 mg/L |
| pH | 6.5–8.0 | <6.0 or >9.0 |
| KH (carbonate hardness) | >4 dKH | <2 dKH |
| Dissolved oxygen | >6 mg/L | <4 mg/L |
The nitrogen cycle explained simply
Fish waste produces ammonia (toxic). Beneficial bacteria convert ammonia to nitrite (still toxic). A second group of bacteria converts nitrite to nitrate (harmless in low amounts). Plants absorb nitrate. This is why you must wait 4–6 weeks before stocking: you are waiting for both bacterial colonies to establish.