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By Dr. Roddy Conrad, October 16, 2002
By Request of Gene of
Note: This material is provided without reservation, for copying and distributing in any way without any need to consult the author. The only request is not to edit the document without permission of the author, and to request permission to post it on a particular web site from

Introduction: What is an ORP meter?
An ORP meter measures the Oxidation or Reduction Potential of the water. This is a direct measure of pollution index of the water, since more pollution gives a lower ORP reading. As described by Chris Walster, a veterinarian at the Island Veterinary Clinic in Staffordshire, England, in the Summer 97 issue of Koi Health Quarterly,
Put simply, ORP is a measure of pollution. In a well conditioned koi pond you want oxidation reactions to occur, as they indicate the breakdown of water products. Therefore, the higher the ORP level, the less polluted the pond; the lower the ORP level, the more polluted the pond. A low ORP can indicate low dissolved oxygen, high nitrites, or high DOC (Dissolved Organic Carbon), with the DOC promoting the increase of harmful bacteria….A pond with an ORP value below 200 mV will promote growth of slime algae, between 200 and 250 will promote blanket weed or stringy algae, above 250 algae growth will be prevented……As ORP increases, the rate of healing of ulcers increases and at high levels ulcers can be prevented…

Quick Summary of ORP levels as applied to koi ponds:
At readings of below 150, significant improvements to the filtration of the pond should be made as soon as possible to avoid massive fish health problems.
At readings between 150 to 200, fish health will not be marginal, with green water and slime algae a usual occurrence, especially in the absence of UV lights.
At readings between 200 to 250, fish health will usually be okay, but not optimum, and stringy algae or blanket weed will normally be a problem.
Readings between 250 and 400 reflect good to superb water quality, and prevention of fish health problems by water excellent water quality control. The higher end of this range is preferred over the lower end for dependably good fish health and fast fish growth.
Readings between 400 and 450 reflect the use of either potassium permanganate or ozone to increase water quality by addition of a chemical oxidant to the pond to oxidize the various dissolved organic compounds and solid waste materials on the pond bottoms and in the filter systems. Readings in this range usually do not harm the useful bacteria in biofilters if the length of time is less than 30 minutes in this range.
Readings between 475 and 550 reflect active potassium permanganate levels which should kill fish parasites without harming the fish, and quickly oxidize pollutants, provided the fish exposure is only a few hours per week. Water with ORP in this range should not be circulated in biofilters since the useful bacteria may be oxidized significantly.
Readings between 550 and 600 should not be continued for more than 15 minutes because of likely damage to the gills of fish.
Water with readings above 700 will sterilize a system of all life forms in about 10 to 15 minutes.

Things that increase ORP readings and improve water quality:
Factors to increase ORP
Griff Thomasson's list:
1 Flow rates. Pond turn over at 1.5 hours min. The faster the better for higher ORP readings.
2 Aeration, water falls, TTs, spray bars etc. the more the better.
3 More efficient mechanical filtration, with frequent cleanouts or solid dumps, increase ORP readings.
4 Temperature. Cooler temps will generally give a higher orp reading as there is a higher level of dissolved oxygen.
5 Fish load. Lower fish loads give higher ORP readings.
6 Heavy feeding gives lower ORP readings.
7 System Maintenance. Clean out filters. Back wash beads filter, rinse filter media in submerged filters.
8 Dead Spots. Find and eliminate dead spots in pond with low flow since this can accumulate solids wastes which lower ORP readings.
9 Add biofiltration capacity to increase ORP.
Good mechanical filtration systems, with frequent dumping of collected solids, goes a long way towards keeping ORP levels up by removing the solid organic pollutants. There are many different ways to accomplish this goal, with many, many good mechanical filter system designs, and it is not the goal of this document to list all those good mechanical filter designs.
Aeration of the water in waterfalls, or with air stones, or with trickle tower filters increase the ORP level of the water by providing more oxygen to oxidize the organic pollutants such as fish poop.
Higher recirculation flow rates increase the ORP levels of the water in various ways. Better solids removal, more aeration, better biofiltration all go with increased water turnover rates.
Using activated carbon to remove DOC (Dissolved Organic Carbon) increases the ORP levels of the water by removal of pollutants from the water. Here the typical charge is 3 pounds of activated carbon per 1000 gallons, changed out with fresh carbon each 3 months. The usual charge is put into mesh bags, laundry bags for example, some place in the filtration system or the waterfall system or stream system. Or even in the actual pond itself.
More biofiltration increases the ORP value of the pond. Trickle tower biofilters do this better by providing a higher level of oxygen to the aerobic (oxygen loving) bacteria which do the biofiltration. But well designed submerged media filters can also deliver superb water quality with high ORP readings.
Addition of low levels of potassium permanganate to increase the ORP reading to the range between 300 and 450 has been beneficial to ponders who have filtration systems which do not automatically maintain water quality in those ORP ranges as a temporary emergency fix of a fish health problem.
Some ponds use ozone injection with on line ORP controllers to maintain ORP levels at a narrow desired range.

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