Keeping Koi in Aquariums

By JamesM April 18, 2025 0 comments

Koi are primarily kept in indoor aquariums to allow for a clearer view of their graceful swimming, vibrant colors, luxurious patterns, and elegant appearance. It is advisable to use the largest possible aquarium, generally with dimensions of at least 90 cm × 60 cm × 45 cm.

I. Aquarium Filtration Equipment

During the process of keeping koi, various impurities such as uneaten food, fish waste, and metabolic byproducts from aquatic plants often accumulate in the aquarium. These impurities can affect water quality, thereby impacting the growth, development, and even survival of the koi. Therefore, filtration equipment is essential to remove these impurities in a timely manner. Filters work by using a water pump to draw water through filter media and related equipment, where it undergoes a series of physical, chemical, and biological processes to meet rearing standards before the cleaned water is circulated back into the aquarium.

Aquarium Filtration Methods

Filtration is a primary method for maintaining clean and stable water quality in an aquarium and is one of the most fundamental facilities. Depending on different rearing needs and aquarium types, various filtration methods are employed. Home aquariums commonly use three types: mechanical filtration, biological filtration, and chemical filtration.
Filter Selection

Based on their placement in the aquarium and their filtration function, filters commonly used in koi keeping can be categorized into several types: undergravel filters, hang-on-back (HOB) filters, external canister filters, internal filters, and hybrid filters.
Filter Media Selection

The materials used during filtration are called filter media. There is a wide variety of filter media, each affecting water quality differently. Common types include filter floss, activated carbon, bio-balls, ceramic rings, resin, and gravel/sand.

Koi in aquarium

II. Aquarium Temperature Control Equipment

Koi are cold-blooded animals, meaning their body temperature and metabolic rates fluctuate with changes in environmental temperature. Water temperatures that are too high or too low can hinder their growth and development. Therefore, temperature control equipment is necessary for aquarium-kept koi to maintain a stable water temperature within the optimal range for their growth and reproduction, ensuring their healthy development. When the water temperature exceeds or falls below the normal range, the temperature control device will automatically activate or deactivate.

Temperature control equipment mainly consists of heating devices, such as heaters, substrate heaters, and automatic temperature controllers. Another category is cooling devices.

III. Aquarium Aeration Equipment

Aeration equipment, also known as air pumps, comes in various types. For large aquariums or those housing many koi, air compressors or high-power, high-output air pumps can be used. For small to medium-sized aquariums or those with fewer koi, specialized electric micro-air compressors are suitable. Based on the mechanism driving air movement, commercially available air pumps are mainly divided into two categories: electromagnetic vibration air pumps and motor-driven air pumps.

When installing an air pump, it should be placed at a height above the water level of the aquarium or koi pond. Placing a piece of rigid foam board underneath the pump can help reduce noise. The operating time of the air pump generally depends on factors such as water quality, koi density, the size of the koi, the season, and water temperature. Under conditions of good water quality, low koi density, and smaller koi size, the air pump may be used less frequently. Particularly in larger aquariums or ponds with lower temperatures where koi do not show signs of surfacing due to oxygen deficiency, the air pump may not need to be operated at all or may only be run for 2–3 hours before dawn. Conversely, under opposite conditions, it should be used more often. If the air pump runs for extended periods or continuously for several days, it is advisable to turn it off for 1–2 hours at noon on sunny days to prevent damage from overheating.

IV. Aquarium Lighting Equipment

Since the primary purpose of keeping koi is for viewing enjoyment, aquariums must be equipped with appropriate lighting. The placement, material selection, and intensity of the lighting should be determined based on the light requirements of the koi and the desired viewing effect. Typically, indoor lighting sources use incandescent bulbs or fluorescent lamps. Incandescent bulbs consume more electricity and illuminate a smaller area but are convenient to use. Fluorescent lamps are now more widely used, available in specifications such as 15W, 20W, 30W, and 40W. They are usually installed above or at the front top of the aquarium, with brightness sufficient to make the aquarium scenery clear. Currently, ultraviolet (UV) sterilization lamps designed specifically for koi aquariums are available. These provide illumination and also offer sterilization functionality, making them an ideal artificial light source, particularly for small aquariums.

V. Other Aquarium Equipment

Aquarium Decorations

Placing decorations in an aquarium not only creates an aesthetic atmosphere and adds visual appeal but also provides hiding places for koi fry and juveniles. Decorations fall into two categories: artificial and natural. Artificial decorations come in a wide variety, including artificial rocks, pavilions, colored backgrounds, castles, toys, and artificial plants and animals. Natural decorations primarily consist of driftwood, rocks, and aquatic plants, which best showcase natural beauty.
Aquascaping Materials

Commonly used materials for aquascaping in aquariums include sand, stones, and miniature crafts.
Auxiliary Keeping Equipment for Aquariums

Common auxiliary equipment for aquariums mainly includes water storage buckets, basins, rubber or plastic tubing, glass pipettes, nets, scoops, plankton nets, temporary holding tanks for live food, feeding racks, tweezers, algae scrapers, magnetic cleaners, cloths, measuring tools, and plant substrate tools.

VI. Stocking Density of Koi in Aquariums

Based on practical experience, the stocking density of koi varies depending on the size of the aquarium and the size of the koi. Table 1 lists the recommended stocking densities for common aquarium sizes and koi lengths, provided for reference.

Note: A dash (-) indicates that stocking koi of that specific length is not recommended for the given aquarium size. These figures are general guidelines. The actual stocking density should be adjusted based on factors such as filtration capacity, frequency of water changes, and feeding regime.

Table 1: Stocking Density of Koi in Different Aquarium Sizes

Aquarium Dimensions (cm)				Recommended Stocking Quantity by Koi Length
Length	Width	Height	5 cm	10 cm	15 cm
36.3	24.2	24.2	2	1	-
39.4	24.2	30.3	3	1	-
39.4	30.3	30.3	4	2	-
45.5	24.2	30.3	4	2	1
45.5	30.3	30.3	5	2	1
60.6	30.3	30.3	6	3	2
60.6	36.3	36.3	10	5	3
90.9	45.5	45.5	20	10	4
90.9	45.5	60.6	25	12	4
121.2	60.6	60.6	45	22	6
151.5	75.4	75.4	80	40	10
201.2	100.3	100.3	200	90	18

VII. Husbandry Management

1. Feeding

When keeping koi in a home aquarium, live feeds such as daphnia and tubifex worms are ideal. Koi fry should be fed with "infusoria." "Infusoria" is a term used among ornamental fish hobbyists, referring to water from lakes, ponds, or ditches rich in protozoa like paramecium. When plankton proliferates in large numbers, it appears as grayish-white clouds drifting in the water, hence the name "infusoria" or "gray water." Nutritionally complete artificial formulated feeds can also be used.

Feeding must strictly adhere to a fixed schedule and quantity to maintain clean water quality. Generally, feeding 1–2 times per day is advisable, preferably in the morning and evening. The evening feeding should occur early, ideally immediately after returning from work. If rain or a temperature drop is forecasted for the evening, reduce or skip feeding. Morning feeding can be done before leaving for work. After each feeding, determine the amount for the next feeding based on whether the koi finish the food within an hour and by observing fecal color to assess digestive health.

2. Waste Removal

Keeping koi in an aquarium requires clear water and healthy fish, making water clarity a top priority. During hot summer months, waste removal should be performed daily, while the interval can be extended in other seasons. The method involves using a rubber hose to siphon out koi feces and leftover food settled at the bottom of the aquarium, then gradually adding an equal amount of aged fresh water to replace the removed dirty water. An efficient filtration system helps maintain good water quality. However, over time, the filter media can become clogged, reducing filtration speed and efficiency. Organic debris trapped in the filter can decompose, producing toxic gases like H₂S, which pollutes the water. Therefore, regular maintenance of the filtration system is essential. If gravel or sand is the primary filter media, it should be washed periodically to ensure proper water flow, and accumulated debris on the filter media should be removed as needed. If activated carbon is used, its lifespan depends on the aquarium's water quality and the carbon's quality, requiring replacement every 2–5 months.

3. Water Quality Monitoring and Water Changes

Water quality monitoring is an essential aspect of indoor koi husbandry. Based on measured data and observed changes in water quality, appropriate management measures can be taken to prevent disease outbreaks and mortality. Routine monitoring should include parameters such as water temperature, pH, salinity, dissolved oxygen, ammonia concentration, nitrite, nitrate, and bacterial count. If possible, targeted testing for certain heavy metal ions should also be conducted. Water changes are a simple and direct method for adjusting aquarium water quality. Changing water is a crucial part of daily management and a key factor in successful koi keeping, hence the saying "raise water first, then koi." The volume, frequency, and schedule of water changes depend on factors such as the aquarium's conditions (mainly the efficiency of the filtration system), the species kept, stocking density, and husbandry methods. The method and steps are as follows:

(1) Water Aging: One or two days before a water change, age the water by filling a cement tank or an empty koi pond at the same location with tap water and let it sit for 24–48 hours until its temperature matches that of the adjacent koi pond. Aging water helps increase oxygen levels and allows chlorine to dissipate. During spring, summer, and autumn, the new water temperature should ideally be 0.5–1.0°C lower than the old water. In winter, the new water should be 0.5–1.0°C warmer.

(2) Siphoning Waste and Removing Decorations: First, use a rubber hose to siphon the clear upper layer of water into a clean container. Then, remove decorations such as aquatic plants and rocks, and rinse them thoroughly with clean water.

(3) Removing the Koi: Use a koi net to catch the koi, covering the top of the net with a scoop to prevent the koi from jumping out and getting injured. For small koi, it is better to use an aluminum scoop to transfer them along with some water into the siphoned clear water.

(4) Cleaning the Aquarium: After removing discolored or foul-smelling substrate, rinse the aquarium repeatedly with clean water and wipe it down.

(5) Restoring the Aquarium: Once cleaning is complete, restore the setup by laying the substrate, placing rocks, adding water, planting aquatic plants, and reintroducing the koi in the correct order. Finally, top up with the aged fresh water to the original water level. This restores the aquarium to a clean and refreshed state.

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