Volume & stocking
A pond is so many litres of water with so much surface. Litres decide how concentrated waste gets and how slowly the temperature moves. Surface decides how fast oxygen and heat are swapped with the air. The weight of fish decides how much waste there is.
Litres, depth and surface
Each fish space is a fixed amount of water, so a capacity upgrade makes the pond bigger, not just more crowded. Depth sets the surface area:
litres = spaces × water per space · surface (m²) = litres ÷ 1000 ÷ depth
A capacity upgrade adds fresh water. Everything already in the pond is diluted straight away, and the bacteria colony grows into the new walls over the next couple of weeks.
What scales with what
| System | Litres | Surface area | Fish weight |
|---|---|---|---|
| Ammonia, nitrate and murk (mg/L) | ÷ litres | where they come from | |
| Oxygen | ÷ litres | surface exchange Planned | koi breathing |
| Natural bacteria | walls and floor | ||
| Water change and salt cost | ✓ | ||
| Filter turnover | ÷ litres | ||
| Temperature: how fast it changes | thermal mass | heat loss | |
| Heating cost Planned | warming up | holding it warm | |
| Food, waste | ✓ |
How much a koi weighs, eats and makes
Weight grows with the cube of length:
weight (kg) ≈ 0.000015 × length (cm)³
Everything else follows weight. A koi in warm water eats about % of its weight a day; each kilogram of basic pellets eaten makes g of ammonia and g of solids.
At 24°C on basic pellets, Normal portion. Fry in the fry pond eat a much bigger share of their weight (%), but they weigh so little it hardly shows.
How many fish?
A pond has two limits:
- Space: the number of fish the capacity allows. You can’t move a fish into a full pond.
- Load: how much ammonia the fish make against how much the bacteria can clear. The Farm tab shows this as the bacteria colony’s busy figure. Over 100%, ammonia builds up whatever you do, until you add filtration, feed less, thin the stock or change water more often.
So a pond of ten tosai and a pond of ten 60 cm adults are completely different ponds. Try it:
Plants add a little more room for bacteria. Low oxygen or cold water make the same colony work slower.