Ammonia & nitrate

Ammonia

Koi breathe ammonia out through their gills all day, in proportion to what they eat. Rotting sludge and dead fish add more. The game measures it like a test kit, as total ammonia in mg/L.

• Sickness risk rises with ammonia above 0.5 mg/L, on top of the overall water score.
• Above 0.5 koi eat less; above 1 they lose health every day and can die. The diary says when ammonia killed a fish.
• Sick fish only recover while ammonia is below 0.5 (and oxygen above 5).
• White foam that won’t pop collects at the edges of the pond when ammonia is high.

Nitrite

In a real pond the bacteria work in two steps: ammonia to nitrite, then nitrite to nitrate. Nitrite is toxic too. A pond whose colony can turn ammonia into nitrite but not yet nitrite into nitrate gets a nitrite spike, usually a few weeks into a new filter.

The game folds the two steps into one, so the Ammonia bar stands for both. A separate nitrite spike for brand-new filters is Planned.

The bacteria colony

Nitrifying bacteria live on surfaces: the pond’s walls and floor, plant stems and roots, and above all filter media, which is built to have as much surface as possible. Two numbers matter:

• Media: how big a colony the pond can host, in grams of ammonia a day. The pond itself hosts . Plants add . Filters add the most: .
• The colony: how much of that room is filled. It grows to match what it’s fed, by up to % of its media a day in warm water, so a new filter takes about three weeks to mature. When there’s less food it shrinks by % of the gap a day, but never below a quarter of its media.

The colony only works at full speed when it’s warm and has oxygen:

ammonia cleared a day = colony × warmth × oxygen

The spring spike. Through winter the koi stop eating, so the colony shrinks. In spring the koi start eating while the water is still cool and the colony is small and slow. Ammonia can climb for a few weeks until the bacteria catch up. Wheatgerm, a Light portion and a water change or two get you through it.

Nitrate

Every gram of ammonia the bacteria clear becomes about g of nitrate. Nitrate isn’t toxic the way ammonia is, but it builds up steadily in a well-fed pond.

• Nitrate is part of the water score: above 40 mg/L it slowly pulls growth and colour down.
• It feeds algae: the more nitrate, the faster the water goes green in sunny weather.
• It leaves through water changes, plants, algae (which eat it), and the mud of a fry pond, where bacteria in the earth break it down.

Plants work hardest in summer and almost stop in winter. Fry ponds lose % of their nitrate a day to the mud; other ponds %.

Water changes

A water change swaps a share of the pond for clean tap water, so it removes that share of everything dissolved or floating: ammonia, nitrate, murk and algae. It also stirs up a little sludge, and fresh water carries oxygen.

• Because it removes a share, a change does most when the water is dirtiest. A 25% change takes 1 mg/L of ammonia down to 0.75, and 0.2 down to 0.15.
• Cost is the water and dechlorinator: per 1000 litres plus for the effort. A bigger pond costs more to change.
• Spring rain and autumn storms count as small water changes.
• In a heated pond, cold tap water also cools the pond and costs energy to warm back up. See Temperature & heating.

Try a pond

This runs the game’s own water model day by day through a summer and shows where the water ends up.

Assumes good oxygen (8 mg/L), a clean filter and no events. Starts from a fresh pond on the first day of spring.