The Specification I Would Change Before Approving the Order
For adult koi, a conventional home aquarium is usually the wrong permanent system. Koi are domesticated common carp ( Cyprinus carpio ), and FishBase records common carp reaching 120–129 cm at maximum size, with a maximum published weight around 40 kg and reported lifespan up to 38 years . FishBase's aquarium guidance also specifies a tank length above 200 cm and explicitly says the species is not recommended for normal home aquariums.
For procurement, I would therefore divide “koi aquarium” into two completely different Products :
Temporary juvenile display/holding tank: possible with careful stocking and oversized filtration.
Permanent adult koi system: preferably a purpose-built pond or very large indoor koi tank measured in thousands of liters , with substantial floor area, mechanical solids removal, biological filtration, continuous aeration, circulation, drainage and backup equipment.
The mistake is buying a 300-liter aquarium for six 10 cm koi because six 10 cm koi fit in it today.
They are not six 10 cm fish forever.
The Length on the Drawing Bothers Me More Than the Liters
Suppose an importer sends Ruibit two concepts.
One is a tall 1,500-liter glass display only 150 cm long.
The other is a much longer, wider system of similar volume.
For koi, I want the second drawing.
Common carp are large-bodied, active fish associated naturally with large lakes, rivers and other substantial water bodies. FishBase describes them as favoring large, slow-flowing or standing waters and records an aquarium length recommendation above 200 cm.
That makes footprint and swimming length more useful than chasing volume by increasing water height.
For a commercial indoor koi display, I would generally begin the engineering discussion around:
| Intended Use | Practical Design Direction |
|---|---|
| Small juveniles, temporary retail holding | 500–1,000 L+ , depending heavily on fish number, size and turnover |
| Larger juveniles / temporary display | 1,000–2,000 L+ , preferably 200 cm+ long |
| Several growing koi | 2,000–5,000 L+ with serious filtration |
| Permanent adult group | Purpose-built large indoor tank or pond; several thousand liters should be considered the starting territory, not a generous upgrade |
These are design ranges, not biological minimums . Stocking density, fish size, feeding rate, water exchange, filtration capacity and duration of holding can change the answer dramatically.
A dealer holding juvenile koi for seven days has a different problem from a hotel planning to keep the same koi for ten years.
Stop Counting Fish. Start Counting Biomass and Feed.
This is where koi systems separate themselves from ordinary ornamental aquariums.
Ten tiny koi can look like a light stocking load.
Then they grow.
Fish biomass increases much faster than body length suggests, feeding increases, and the filtration system receives the bill.
The nitrogen cycle does not care that the aquarium still contains “only ten fish.”
For a Ruibit commercial koi project, I would therefore ask the buyer for:
number of koi;
current average length;
expected adult size;
daily feed quantity;
temporary or permanent holding period;
target water temperature;
and maximum planned biomass .
Then I would size filtration around that load.
This is more defensible than rules such as “one koi per X gallons,” because a 12 cm juvenile and a 70 cm adult clearly do not represent the same biological load.
The First Equipment I Want Is a Solids Filter
Koi produce a lot of physical waste.
I do not want that waste entering expensive biological media first.
The filtration train should therefore begin with effective mechanical solids removal .
Depending on system size and commercial requirements, that may involve settling, brushes, screens, filter socks, drum filtration or other appropriately designed mechanical stages.
The principle is more important than the specific device:
Remove solids early. Keep the biological stage clean enough to breathe.
A compact aquarium filter designed around tropical ornamental fish can become a maintenance nightmare when applied to heavily fed koi.
For procurement, ask:
How quickly can the first mechanical stage be cleaned?
If the answer involves dismantling half the system, somebody eventually stops cleaning it on schedule.
That is not a maintenance problem.
It is a design problem that has been waiting patiently.
Biological Filtration Has to Follow the Feeding Load
After solids removal comes the biological stage.
Nitrifying bacteria convert ammonia generated by the fish into nitrite and then nitrate. The larger the fish and the heavier the feeding, the more biological treatment capacity the system needs.
This is why I would not accept:
“Large biofilter included.”
on a koi-system quotation.
Specify:
media type;
actual media volume;
filter chamber volume;
designed water path;
aeration where required;
and the biological load the supplier used for sizing .
FishBase records common carp as omnivorous feeders consuming benthic organisms and plant material. Large koi kept and fed intensively can therefore represent a substantial continuous waste load.
The biological filter needs to be designed for the koi after lunch.
Not for the empty tank photographed before shipment.
Oxygen Is Equipment, Not an Optional Accessory
For a large koi system, I want dedicated aeration.
Not merely surface movement from the return pipe.
Use air pumps, diffusers or another engineered oxygenation method appropriate to the system. The biofilter also consumes oxygen while processing nitrogen waste, so fish and bacteria are drawing from the same oxygen budget.
And I would consider redundancy.
Two air pumps at sensible capacity can be more useful than one oversized unit if losing the single unit means losing all aeration.
For valuable koi, backup power deserves the same discussion.
A beautiful filtration system without circulation and aeration during an extended power failure becomes a container full of fish and increasingly bad water.
Return Pump: Ignore the Zero-Head Number
The pump should be selected from the real hydraulic system .
If a pump says 15,000 L/h on the box but must lift water 1.8 m through narrow pipe, several elbows and a filter train, the delivered flow will be lower.
So a commercial RFQ should state:
required delivered flow at design head , not simply nominal pump flow.
I also like redundancy in permanent systems.
For example, two appropriately sized pumps can allow partial circulation if one requires service.
Whether that architecture is worthwhile depends on the livestock value and consequences of downtime.
A retail tank of inexpensive juveniles and a collection of high-value Nishikigoi should not automatically receive identical redundancy.
The Equipment List Gets Long for a Reason
For a properly engineered large koi aquarium or indoor holding system, I would expect to discuss:
Mechanical Filtration
High-capacity, easily cleaned solids removal.
Biological Filtration
Sized from biomass and feeding rather than tank volume alone.
Circulation Pumps
Selected at actual operating head, with redundancy considered for permanent/high-value installations.
Dedicated Aeration
Air pump, diffuser system and adequate oxygen distribution.
UV Clarifier
Useful for controlling free-floating algae and helping manage water clarity; it does not replace biological filtration.
Drainage and Water-Change System
A bottom drain or other effective low-point waste collection becomes particularly valuable in large installations.
Design water changes so staff do not need to carry buckets around a 3,000-liter system.
Temperature Monitoring
Common carp tolerate a broad temperature range; FishBase reports 3–35°C for the species, but that biological tolerance should not be confused with an ideal operating range for a specific koi collection.
Heating or cooling should therefore be specified according to climate, indoor conditions and husbandry objectives.
Water-Quality Testing
At minimum:
ammonia, nitrite, nitrate, pH and temperature.
Depending on source water and system operation, alkalinity and dissolved oxygen monitoring can become particularly useful.
Backup Equipment
Spare/secondary aeration, circulation strategy and power-loss planning should reflect livestock value.
This is the equipment customers rarely ask about when everything is running.
It becomes the equipment they care about most when something stops.
A Glass Koi Tank Creates a Structural Problem Too
There is another reason Ruibit cannot treat a permanent koi aquarium like a scaled-up 200-liter fish tank.
Several thousand liters of water means several thousand kilograms of static load before glass, frame, filtration and cabinetry are counted.
3,000 liters of water alone is roughly 3,000 kg.
Then consider panel span, water height, glass thickness, deflection, silicone joints, bottom support and floor loading.
For a large indoor koi display, structural engineering and life-support engineering have to meet on the same drawing.
I would rather make a koi tank longer and wider than unnecessarily tall where the site permits it. The fish gain useful horizontal space, while extreme water height increases glass loading and structural demands.
Liters are not free.
They push on something.
What I Would Put on the RFQ
For a hotel, koi dealer, garden center or high-end residential project, I would send the manufacturer something closer to this:
Application:
permanent display / temporary holding / retail
Fish count:
___
Current koi size:
___ cm
Maximum design size:
___ cm
Target daily feeding:
___ g/day
Tank dimensions:
prioritize long, wide footprint
Mechanical filtration:
specified solids-removal stage
Biological filtration:
media type and volume declared
Aeration:
dedicated, preferably redundant where justified
Pumps:
delivered flow stated at actual design head
UV:
sized for actual system flow and intended function
Drainage:
bottom/low-point waste removal + easy water change
Water monitoring:
ammonia, nitrite, nitrate, pH, temperature
Emergency design:
circulation/aeration backup
Structural documentation:
glass, frame, support and foundation/floor-load review
Maintenance access:
every pump and filter component removable without dismantling the display
I also searched the supplied Ruibit Alibaba storefront while preparing this article. I could not verify enough publicly indexed model-level koi-tank specifications to responsibly assign a particular Ruibit SKU, glass thickness or filtration capacity, so those values should be confirmed against the actual Ruibit project drawing and quotation rather than invented here.
Design for the Koi You Will Own, Not the Koi You Bought
This is where koi catch buyers out.
A 15 cm koi looks reasonable in an aquarium.
A group of them looks even better.
But common carp can become very large and very long-lived; FishBase records maximum lengths around 1.2–1.29 m and reported ages reaching 38 years .
So I would not sell a permanent koi system by asking:
“How many liters do these fish need today?”
I would ask:
“How large are you prepared for these fish to become?”
For juvenile retail holding, a large engineered aquarium can make sense.
For permanent adult koi, think beyond the conventional aquarium: long footprint, thousands of liters, heavy-duty waste removal, serious biological filtration, dedicated oxygen and a maintenance system designed for years rather than weekends.
The glass tank is only the visible part.
With koi, the real product is the water-treatment plant attached to it.
