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…