10 · Fish & Stocking
Fish add life and motion, but they also add load. The single biggest mistake is overstocking.
Stocking density (rule of thumb)
goldfish: ~1 adult per 150–250 L of water
koi: ~1 adult per 1000 L (they grow huge & are messy)
wildlife pond: 0 fish — they eat tadpoles & larvae
Species choices
| Species | Traits | Notes |
|---|---|---|
| Native minnows / shiners | Small, lively, ecological | Great for wildlife-style ponds |
| Goldfish / comet | Hardy, colorful | Do not release into wild — invasive |
| Koi | Large, personable | High bioload; needs big, deep, filtered pond |
| Orfe / Tench | Surface / bottom foragers | Peaceful community fish |
| Stickleback | Tiny native | Excellent for small wildlife ponds |
Never release
Do not "set free" pond fish into local streams, lakes, or canals. Goldfish and koi become invasive and devastate native ecosystems. Rehome responsibly.
Feeding
- Feed only what fish eat in 2–3 minutes, once daily in warm months; stop when water drops below ~10°C.
- Overfeeding is the fastest route to ammonia spikes and algae blooms.
- In a planted pond, fish often need little supplemental food — they graze naturally.
Acclimation
Float the bag in the pond 15–30 minutes to equalize temperature, then gradually add pond water to the bag before releasing. Never dump bag water into the pond (it carries transport waste).
Stocking procedure (do not skip steps)
- Wait 4–6 weeks after filling before adding any fish. The biological filter needs time to colonise.
- Test water: ammonia and nitrite must read 0, pH 6.5–8.0.
- Start with 2–3 hardy fish (goldfish or weather loach).
- Float the bag for 20 minutes to equalise temperature.
- Gradually mix pond water into the bag over 30 minutes.
- Release fish and do not feed for 24 hours (stress reduces appetite).
- Wait 2 weeks before adding more fish, and test water again.
Never add fish to a new pond immediately
Even a small amount of ammonia from the liner, concrete, or soil will burn fish gills. The nitrogen cycle must establish first. Patience prevents fish death.
Stocking density guideline
1 cm of fish per 10 litres of water. A 5,000-litre pond can support roughly 50 cm of total fish length. Overstocking is the most common cause of fish disease and poor water quality.