The mud pond
Japanese breeders raise their fry in mud ponds, doro-ike: open earth and clay ponds with no filters at all. The pond’s own life does the work. A bloom of plankton feeds the fry and takes up their waste, clay binds what’s left, and fresh water runs through from the hillside. The fry pond in Nishiki Pond works the same way.
How a mud pond stays clean
- The plankton bloom. Green phytoplankton and the tiny animals that eat it (daphnia, larvae) are the base of the pond’s life. The bloom feeds the fry and takes up ammonia as it grows: g a day per m² at full bloom.
- The mud. Bacteria in the earth clear ammonia ( g a day per 1000 L) and break nitrate down (% a day), better than any lined pond.
- Shallow and wide. The surface takes in oxygen × as fast as a deep lined pond’s.
- No filter, no vacuum, no air stones. None of them belong in a mud pond. Solids settle and break down in the mud; raking helps them along.
Real ponds work on the same ideas: heavy clay that binds toxins and releases minerals, a plankton bloom started with manure before the fry go in, mountain water running through, aeration when it’s needed, and feeding cut back when the water struggles.
Sizes
Each fry space is litres of water, m deep. Capacity sets how many fry a spawn gives: they fill whatever room the fry pond has.
Natural food and fertilising
Fry eat the bloom first, and the keeper cuts the pellets to match, so a good bloom means fewer pellets, less waste and better-fed fry. Whatever food is in the feeder still counts: growth pellets still grow fry faster on top.
- A full bloom offers g of natural food a day per m² in warm water. Small fry live almost entirely on it; by late summer they’ve outgrown it and pellets take over.
- Fertilise in spring, by day : manure costs . The bloom builds over about a week, holds for days, then fades over more. You get a reminder on spring day .
- Miss it and the pond only makes % of a bloom by itself: smaller fry, more pellets.
- The plankton culture does it for you every spring on day , for × the price of the manure.
Hands-on care
Upgrades
Clay and the inflow channel are winter work: the pond has to be drained. The paddlewheel, net cover, feeder and capacity can go in any time.
How hard can you push it?
Tested over a summer with 25 fry, fertilised in spring, raked whenever the solids leave “Clear”:
| Feeding | Kit | What happens |
|---|---|---|
| Basic pellets, Light | none | Stable all summer. One or two rakes from about summer day 23. |
| Basic pellets, Normal | none | Usually fine; some years it clouds over late in summer and needs regular raking. |
| Growth pellets, Normal | none | Bigger fry, but ammonia can climb towards 1 mg/L and the water turns Poor. |
| Growth pellets, Normal | clay bottom, spring inflow | Back to Good, with the sluice opened every few days. |
| Growth pellets, Heavy | clay, inflow, paddlewheel | Workable, but needs constant raking and flushing. |
Faster-growing fry, more fry and hungrier lines all push the same way. Push harder, fit more.