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Article by Mr. Eric Lim of Singapore Koi Club

By: Eric Lim

Many a times, new koi hobbyists got caught with the love of nishikigoi. Not knowing that koi do create lot of waste & need frequent maintenance if the filter system is not done correctly, most proceed to get a easily / cheaply done pond / filter & then proceed to buy expensive koi – only to see one by one dying off.

The most common problem is due to wrong design of the filter system (especially the size of drain line), making cleaning of the first few chambers tedious. So owner clean less often, causing water quality to go bad, then koi getting sick – And after 2 yrs. of frustration - Owner gave up!

The objective of this article is to help new hobbyists to plan a good working filter system and other useful tips, so as to make koi keeping an enjoyable hobby and not be slave to the hobby.

By recommending a near 13tons pond, this I felt is the minimum that one can go, especially keeping out-door in our tropical weather. Remember that water temperature can rise a lot in the hot afternoon sun. Also,  green algae will be a problem if the capacity of the filter is insufficient. Actually for this problem, top shading is a must to control algae growth. Most of our advance club members’ ponds have top full shading. But remember to use strong wooden structure & non-toxic paint or coating. `Chenghai’ wood is meant for out-door use, can even be use submerge in the filter. All filter top covering (to preserve biological bacteria growth) should also use this.

Another common problem is – many ponds have objects/ pipes sticking out in the main ponds, thereby causing injury to koi when it dash madly during feeding, etc. Sometimes, the injury cannot heal over time and your precious buy becomes a junk koi.

This capacity is not meant to keep jumbo koi. Achieving 70cm growth is possible, with low stock. But best to keep `bonsai’ koi for high stocking. Still, knowing new hobbyists, they tend to over-crowd. Hence the filter system is 40% of the total capacity. Or 60% of the main pond.

The key things to remember are:

  1. No objects sticking out in the main pond to prevent injury.
  2. Top shading is required if exposed to long hours of sunlight. Will reduce much algae growth.
  3. Depth of pond – not less than 5 feet. Best if at 6 feet & more. In the afternoon, surface will heat-up and koi will swim down to stay cool.
  4. Cleaning of CH1 is daily or minimum, 2 days once. Pending the amount of dust, CH2 & 3 may also be as regular. If not – clean weekly.  
  5. Be discipline & keep good water by regular cleaning. The koi will stay healthy & take care of itself.  
  6. Size of drainage (here its 4ins.) – larger means drain faster, you clean happier & more regularly. Don’t let the contractor out-talk you into a smaller size, to save budget. Think long terms. At least 3” minimum.  
  7. Biological bacteria do not establish after 3 feet depth. So no point having a deep filter. Shallow depth makes it easier to view  & clean.  
  8. Electrical Points – best to install different circuit breakers for both water pumps as most cheap mechanical-seal pump only has a life span of 2 to 3 yrs. If one trip causing all power supply to stop and no one is at home for long hours – expensive mistake. If you only have one water pump & one air pump – also use two CB points. Koi will not die as far as there is air circulation in the main pond. Air pump does not trip. UV unit needs electricity too.  
  9. Pump turnover rate – the total capacity should not be less than once / hour.  Present design using two OMU-2 pumps (total capacity = 26,400 liters /hr.) is 2 times / hour.  
  10. Back flushing from P2 – use this for back cleaning of bio. chambers. Do not use tap water to back wash.  
  11. If pH is low (6.5 or <), convert CH 6 to hold coral chips in holding bags for easier cleaning. Dirty chips not wash can caused `Hikkui’ disease.  


  1. To clean CH1, pull out PP No.1 (pull pipe), plug into 4” inlet (to stop flow) & pull out BP1 (by-pass) to continue the filter flow. Pull out PPA to drain to dry sump. Pull out PPF to drain to sewer.  

  2. To clean CH2 & 3, pull out PP No.2 & 3. Pull out BP1 (by-pass) to continue flow. Pull out PPB to drain to dry sump & PPF to sewer.  

  3. To clean CH4, 5 & 6. Pull out BP3 (by-pass) to continue pumps outlet. Pull out PP4 & PPA for CH4 and PP5, 6 & PPE & D for CH5 & 6 to drain to dry sump. PPF  - to sewer.  

  4. To drain pond bottom, pull out PPC & PPF to drain to sewer.  

Overflow points – As no pipes are to protrude out, all up-turn inlet elbows are recess into the filter chambers.

OF No.1 (over-flow) – is set at just below the usual water level. This is a continuous flow, to skim & maintain the water surface sparking clean. OF 2 is  lower by 3in than OF 1, for by-pass use.

OF 3 – is set higher by 2 inches from OF 1. In the event oily or dusty surface was encountered, top-in tap water to increase the height & continue a flow to skim out the scum. OF 1 is shut-off.

Converting CH No.1 chamber into a hospital/ quarantine tank – Pull out BP 2 (by-pass). OF 2 short pipe swap over to cover OF 1. Insert a 4” pull pipe to close up the inlet. See Note 1 & 2 = the heights are different. By dropping usual water level, CH 1 is isolated by itself. Inlet now starts at BP 2 (CH 2).

Medicating/ isolating the main pond and convert filter system to self-circulation – Pull out BP 3 & plug up CH 1 inlet. Drop usual water level till below CH 6 outlet level. Disconnect water pump P 2 and put into CH 6. Run a hose to CH 2 and start P 2 circulation.

This is meant to be a last resort in situation where all koi are infected. Always practice the habit of quarantine all newly purchases and to catch & treat any sick/ injured koi ASAP, outside.

Note: If Dry Sump is lower than outside sewer, a powerful water pump with Auto-Low Level cutout must be used to push out the drained water fast.

Filter Pump use specification;

Model : OMU-2 (cost approx. S$250+)

Max cap. 220 liters/min (13,200 l /hr)

Head/Max : 5.2m     Wattage : 150W  


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