山口県宇部市から情報発信 Information from Ube City, Japan



Biofilter(1) – Water purification for ornamental fish aquarium


Table of contents

I. Water treatment by biofilm method
   Functions and Features, Reaction Tank
   Bio-carrier and Selection
   Application Fields and Examples
Ⅱ. Experimental and Understanding of Biofilm Process
 1. Ornamental Fish Aquarium (this page)
 2. Persistent Industrial Organic Wastewater
 The management of a ornamental fish aquarium is very helpful for beginners in understanding water purification using microorganisms. Here, the outline of the method for keeping the small aquarium of ornamental fish will be described.
 Visitors with more specialized interests may refer to the other important pages linked on this site.


 1) 水槽(約幅60×奥行30×高さ36cm/約60L)、2) 上部ろ過器(またはフィルター)(水槽幅60cm用)、ポンプは浸漬型が望ましい, 1~2台、3) ろ過材(大磯砂 細目、3~5mm程度)、4) 酸素補給用のエアポンプ(ダイヤフラム式)・エアーストーン(曝気器具:60L水槽用)、5) 温度調節用ヒーター(熱帯魚飼育の場合、固定温度式100~200W程度でよい)。

1. Aquarium and equipment to be prepared

 The aquarium and equipment used can be obtained from specialty stores, home centers, or the Internet. Aquariums come in various sizes, but the standard size is 60cm wide and 60L capacity. a set of aquarium and equipment of this size is available at an appropriate price, and would be appropriate for deepening understanding of biological purification system. The size smaller than this is not so useful as the biological filter is small. If it is more than this, the cost of the aquarium, equipment, food, electricity, etc. will be high, and the location will be a spacial problem. The aquarium and necessary equipment required for the experiment are shown below.
 1) Aquarium (approx. width 60 x depth 30 x height 36 cm, approx. 60L), 2) upper biofilter or filter (for water tank width 60cm), one or two units of pumps are preferably immersion type, 3) filter material (sand gravels, diameter of about 3 to 5 mm), 4) air pump (diaphragm type), air stone (for 60L aquarium), 5) temperature control heater (fixed temperature type of 100 to 200W for tropical fish breeding is enough).
 It is appropriate to install a filter above the aquarium because it is easy to observe and maintain. Various types of filter media (the part of filled gravels is called filter bed) have been developed and marketed, but sand gravels (around 3 to 5 mm) is most suitable for the purpose of this water purification experiment. It is convenient in terms of management to use a removable container for holding the filter medium (the bottom of which is shaped like a slats). The water pump for circulation through the filter is a submersible type (impeller type driven by magnet rotor) that has excellent durability (no bearing, no lubrication) and excellent noise reduction.
 Although not required, a small submersible pump for draining water when exchanging water (spout volume: about 8L/min, used when pouring bath water into a washing machine, etc., available at a home center (an example, which color is pink and conected pink tube, is shown at the bottom left of Photo 1).



2. Preparation before fish breeding

 Put the gravels in a bucket, stir and wash. Tilt the bucket to remove the wash water and repeat this several times to clean the gravels. Spread the gravels around the bottom of the aquarium to a thickness of 3 to 5 cm. The main body of the biological filter is filled with gravels up to about 70 to 80% of the height of the filter medium container.
 Fill water into the aquarium until the water level indicated on the immersion pump is reached. Turn on the water pump and let water pass through the filter.

写真1 鑑賞魚水槽の準備  Photo 1 Preparing the ornamental fish tank.
Biofilter: Fill sand gravels of about 80% of the container depth, and bottom sand gravel layer: thickness about 5cm.



3. Microbial breeding

 Immediately after the setup, there are no microorganisms for purifying water, so microorganisms are transplanted to the filter bed and the gravel bottom layer in the aquarium. Some of the methods are follows.
 (1) Skim milk powder and about one or two cup of spoon are added daily for several weeks to breed the microbial community. Immediately after the start of operation, water is polluted, but if it is cleaned with the passage of days, it is ready.
 (2) Add about 1 to 1.5 L of returned sludge from the sewage treatment plant to the aquarium.
 (3) About 1L of soil such as farm field is transferred to a bucket, stirred, the settled suspension is sloped and collected with draining water, and transferred to an aquarium.
 (4) Transfer about 1L of bottom sands or mud from river (freshwater fish) or coast (seawater fish) to a bucket, collect by the method of (3), and transfer to an aquarium.
 In the method of (2) to (4), if the suspension (microorganism group) is fixed and retained in the filter bed and bottom gravel layer within half a day to one day, and the water in the aquarium is clear, the breeding fish are OK to be immediately transferred. All preparation is complete.


 魚のサイズ及び飼育数、給餌などに関しては、他書やインターネットから情報を参考にするとよい。金魚、ヒブナ、オイカワ(河川で採取、Photos 2と3)、中小サイズ熱帯魚(専門店・インターネットから購入)など、淡水魚が飼育し易い。熱帯魚や海水魚も興味ある観賞魚であるが、大型に成長する魚類は餌・水槽に課題があり、初心者には向かない。海水魚では水の冷却装置が必要である。観賞池生け簀料理店用の大型水槽の設備・水質管理については、別途解説している。

4. How to raise fish

 Regarding fish size, number of breeding, feeding, etc., it is recommended to refer to information from other books and the Internet. Freshwater fish such as goldfish, Hibuna, Oikawa (collected in rivers, see Photos 3 & 4) and small- and medium-sized tropical fish (purchased from specialty stores and the Internet) are easy to breed. Tropical fish and seawater fish are also interesting ornamental ones, but large-sized fish to grow up are not suitable for beginners due to problems in food and aquarium. A seawater fish needs a water cooling system. Facilities and water quality management for large aquariums, ornamental ponds and fish tanks at markets and restaurants are explained separately.

写真2 淡水魚飼育事例-ヒブナ
Photo 2 Freshwater fish breeding case – Hibuna
写真3 淡水魚飼育事例-オイカワ
Photo 3 Freshwater fish breeding case – Oikawa



5. Maintenance of biological filter

 When the microbial group retained in the filter bed grows and the filtration resistance in the bed increases, circulating water overflows from the partition plate of the filter container. The water and microbial group in the filter bed do not contact each other and the purifying function is no longer fulfilled. Transfer the bed gravals to a bucket, inject tap water, stir and exfoliate the microbial flocs that have grown excessively, slope the bucket and discharge the wash water, repeat this washing operation several times, and retun the cleaned gravels into the bed container.
 In addition, biological flocs grow in the gravels at the bottom of the aquarium, and water is suspended when stirred, but it is not necessary to remove the microbial flocs. Immediately after washing the gravels in the filter, the purification function does not work because there are no microbial groups. However, since a large amount of microorganisms are retained in the bottom layer of the aquarium, the purification function is continuously maintained. Furthermore, excess microbial flocs are transferred to and retained in the filter above the aquarium, and the purification function resumes immediately. In particular, since the ammonia-oxidizing bacterium described below has a slow growth rate, the microorganism in the bottom gravel layer is important.


 魚飼育水槽の水質管理で最も重要な事項は、溶存酸素濃度 DOpH温度である。温度については、飼育魚の最適な温度を維持すればよい。
 固形の給餌残渣や固形排泄物(以下、固形有機物)は、底部やろ過器内の微生物によって、分解・可溶化された後、酸化・無機化される。魚から排泄された尿中アンモニア及び固形有機物が分解され生成するアンモニア NH4+は、硝化菌によって硝酸イオン NO3に酸化される。硝化反応によってアルカリが消費されるため、pHの低下が起こる。
 (1)重曹(炭酸水素ナトリウム)NaHCO3または酸化マグネシウム MgO(リンク文献の4.1 一般酸性廃水の中和処理を参照)を添加する。酸化マグネシウムは溶解度が低く、過剰に添加してもpHの急激な上昇がないので、pHの管理には最適である。

リンク文献図-3の上図の(○)に示すように、中和または水交換なしで魚を飼育すると、経過日数の経過とともにpHは低下する。この実験では、20日~30日においてpHが低下し、30日以降はpH 4.7前後で一定となっている。この値以下にpHが低下しないのは、H+ + HCO(aq)
⇄ H2CO3(aq) → CO2(gas) + H2O の反応から分かるように、酸性では溶解度の低い炭酸ガス CO2へ変化し、それが曝気により大気中へ放出されるからである。
 この試験魚はヒブナである。魚種にもよるが、一般に淡水魚はpHの変動(藻類や水草が繁殖している浅池では、夏期・晴天日の午後にはpH 10を超える値まで上昇し、夜間にはpH 5程度まで低下することは珍しくない。)に対してかなりの耐性があるが、海水魚はpH変化への耐性が低い。

6. Water quality measurement and management

 The most important items for water quality control in fish aquariums are the dissolved oxygen concentration DO, pH and temperature. Regarding the temperature, the optimum temperature of the breeding fish may be maintained.
 Dissolved oxygen and pH are important control items for both fish and aerobic microorganisms responsible for water purification. If the air pump works normally, there will be no problem. Most of the supplemented dissolved oxygen is consumed by microorganisms inhabiting the filter bed and bottom gravel layer, not the respiration of fish. The diaphragm of a recent air pump (which sends out air by the combination of the disk vibration driven by an electromagnet and the suction/discharge valves) has high durability and can be continuously operated for about 1 year or more. But when the amount of air diffused from the air stone is low, replace the pump with a new one.
 Solid feed residues and solid excrements (hereinafter referred to as solid organic matter) are decomposed and solubilized by microorganisms in the bottom and the filter, and then oxidized and mineralized. Urine ammonia excreted from fish and ammonia NH4+ produced by decomposition of solid organic matter are oxidized to nitric acid NO3 by nitrifying bacteria. As the alkali is consumed by the nitrification reaction, the pH is lowered.
 Normally, tap water contains hydrogen carbonate ions and has a pH buffering effect, but if all of them are consumed, the pH will drop, the nitrification reaction will not proceed, and ammonia will accumulate. The degree of ammonia toxicity varies depending on the fish species, but the accumulation of ammonia reduces the feeding activity of the fish. Moreover, higher ammonia concentrations lead to fish death.
 There are the following two methods for properly managing pH.
 (1) Add baking soda (sodium hydrogen carbonate) NaHCO3 or magnesium oxide MgO (see 4.1 Neutralization Treatment of General Acid Waste Water in Link Literature). Magnesium oxide has low solubility, and even if added in excess, it does not cause a sharp rise in pH, so it is optimal for pH control.
 (2) About once a week, replace 1/3 of the water in the aquarium with new tap water. This is to replenish the hydrogen carbonate ions contained in natural water.

 As indicated by (○) in the upper part of Fig.3 in the link literature, if fish are raised without neutralization or water exchange, the pH will decrease with the passage of days. In this experiment, the pH decreased in 20 to 30 days, and after 30 days, it became constant around pH 4.7. The pH does not drop below this value because of the reaction: H+ + HCO(aq) ⇄ H2CO3(aq) → CO2(gas) + H2O. As can be seen from the reaction above, carbonic acid changes into carbon dioxide CO2, which has low solubility in acidic water, and is released into the atmosphere by aeration.
 This test fish are Hibuna. Although it depends on the fish species, the pH of freshwater fish generally fluctuates (in shallow ponds where algae and aquatic plants breed, the pH rises to over 10 in the afternoon of summer and sunny days, and to around pH 5 at night. However, seawater fish are less tolerant of changes in pH.
 Comparing (●) in the upper and lower gigures, a large difference was observed in the intake of food, resulting in anorexia. In medaka fish, they become sluggish and do not eat food.



7. Mechanism of water purification by microorganisms

 By the microorganisms inhabiting the bottom surface layer and the filter bed of the filter, organic matter is biologically oxidized by the following processes.
 (1) Solid organic matter (bait residue and excrement) is decomposed and solubilized in a low molecular form by small animals, molds and bacteria.
 (2) Solubilized organic substances (compounds of carbon, nitrogen, phosphorus, and sulfur) are oxidized/mineralized into carbon dioxide, ammonia, phosphate ions, sulfate ions, and the like by aerobic microorganisms. Phosphate ion forms an insoluble compound with calcium and magnesium, and its concentration is low in water.
 (3) Ammonia is oxidized to nitric acid ion through nitrite ion by nitrogen-oxidizing bacteria.
 (4) Organic substances (carbohydrates, proteins, fats, fibrin, etc.) are hydrolyzed into low molecular weight compounds such as monosaccharides, peptides/amino acids, glycerin/fatty acids.
 (5) If the depth of the bottom sand layer is about 10 cm or more, anaerobic microorganisms can live in it, and the above-mentioned low-molecular weight organic matter is anaerobically decomposed (methane fermentation) to produce methane, carbon dioxide, and ammonia and hydrogen sulfide, etc. However, at a depth of about 5 cm, most of the low-molecular-weight organic matter migrates into water and is mineralized by aerobic microorganisms in the surface layer and the filter bed as described in (1) to (4).
 The point to be noted here is that in the process of (3), the conversion of ammonia to nitrous acid has a high pH dependency, and if pH is not maintained near neutral, ammonia accumulates as described above (above). (Shown with (▼) in Fig.3 in the link document of Appendix).


 魚飼育水槽の水質の主な測定項目は、1) 温度、2) 溶存酸素、3) pH、4) 無機性アンモニア化合物(アンモニア、亜硝酸イオン、硝酸イオン)である。1)~4)の項目の測定装置は高価であり、これらの機器が完備されている研究室・実験室では可能である。

8. Water quality measurement and water quality management

 The main measurement items for water quality in quariumss are 1) temperature, 2) dissolved oxygen, 3) pH, and 4) inorganic ammonia compounds (ammonia, nitrite ion, nitrate ion). The measuring devices of items 1) to 4) are expensive, and it is possible in a laboratory equipped with these devices.
 However, it is difficult to measure at home or office. Today, simple water quality measuring kits and instruments are available on the market, and can be obtained from internet sales or specialty stores.
 The appropriate water quality depends on the species of fish to be raised and is not the purpose of this page, so refer to other books, the Internet, etc. for specific numerical values ​​for each item. The point is that the microorganisms responsible for water purification function cleanly if the health of the reared fish (normal/active predation activity) can be maintained.



9. Others

(1) Number of times the gravel in the filter bed is washed.
 Normally, once a month. Two filters are installed in the case where a large amount of food is supplied and cleaning is required every few weeks.
(2) Cleaning the impeller blades of the water absorption pump.
 At the time of cleaning the gravels, the impeller blades of the water pump also scrape off the adhering biofilm with a toothbrush.
(3) Feeding.
 Although it depends on the fish species, generally, the amount of food that is consumed in about 5 to 10 minutes is administered once a day. Floating and sinking, but depends on the fish species. Choose the particle size according to the mouth size of the fish.
(4) Water supply.
 The water in the aquaraum water evaporates and gradually decreases. In the case where a tropical fish is raised and a heater is used, the amount of decrease is severe in winter. Add tap water as appropriate. It is not necessary to remove chlorine in tap water except for special fish species. In winter, if you cover the top of the biological filter and aquarium with a glass or transparent plastic plate, you can significantly reduce the amount of evaporation.


Posted: May 07, 2016
Update: September 07, 2019 (This page was posted early after this site was opened. Since there are many pages that can be linked to important matters, the editor added some corrections to that section. Throughout the full text, there is no modification of the content itself.)
Update: August 05, 2020 (English version added)
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