ORP Meters Applied to Fish Ponds
By Dr. Roddy Conrad,
October 16, 2002
By Request of Gene of www.koivillage.com
Note: Since this document is frequently being revised, no permission
is given to post it on other web sites to avoid obsolete versions of the
document to be found on the internet. There is no objection to printing
and distributing the document at pond club meetings, or similiar events.
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 and 200, fish health will be marginal, with green
water and slime algae a usual occurrence, especially in the absence of
UV lights.
At readings between 200 and 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 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 above 300 are preferred
for healing sick fish.
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, TT's, 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. More small feedings give less
wasted food and higher ORP readings with better fish growth than fewer
large feedings. Ideally, 4 to 6 small feedings a day should be practiced
for optimum fish growth without upsetting the pond filtration system.
7 System Maintenance. Clean out filters. Back wash beads filter, rinse
filter media in submerged filters.
8 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.
A typical example of the low level potassium permanganate treatment option
This low level PP treatment is a way to temporarily increase the water
quality and fish health in problem periods.
Below is a table of an actual example of a ponder, namely Dorothy M in
Massachusetts, using low level potassium permanganate treatment to increase
ORP level to cure sick fish. The long range fix is to improve the filter
system so the water quality stays so high, by ORP reading, that the fish
do not get sick. In this case, Dorothy was unable to catch two small sick
fish in a very large pond, so used this technique to improve water quality
so the fish could heal themselves.
Day No PP additions Total PP added Starting ORP Ending ORP Temp
1 2 .33 PPM 249 261 58F
2 3 .83 PPM 237 467 57F
3 1 .66 PPM 267 416 59F
4 1 .33 PPM 273 446 58F
5 2 .5 PPM 310 481 62F
6 1 .08 PPM 410 468 56F
7 1 .25 PPM 309 473 58F
8 0 0 395
9 0 0 305
10 0 0 379
11 0 0 406
12 0 0 315
13 0 0 318
14 0 0 338
Here Dorothy stopped adding PP on day 8 when the reading stabilized over
300, and the system was clean enough with enough filtration to maintain
the reading above 300 for at least another week in that range, giving
her two small sick koi enough time to heal at high ORP readings. This
is a typical example of good use of low level PP additions to clean up
a pond without bypassing the biofilter. This is a pond with good biofiltration
and aeration, but with marginal mechanical filtration.
Example of using ORP readings and a fish log to systematically improve
the filter system
Griff Thomasson, on the North Carolina coast, took this log of ORP readings
and pond management practices to gradually increase his water quality
to higher levels in a highly stocked koi pond.
Today (October 16, 2002) Griff used a stock solution of PP to raise ORP
readings. A storm last night blew over the huge Umbrella palm and ORP
readings were in the 250 range from the deposits made by the storm in
the koi pond. Griff mixed up a stock solution of 15 grams of PP in one
liter of water and add 200 ml every few hours. ORP never went over 414.
It took 5 treatments to get ORP to stabilize in the 300 range. This has
worked well as Griff does not have a high pond turnover rate. Griff was
surprised that it took so much PP to adjust the pond for the amount of
sediment the storm dropped into the pond.
Pond log of this 8000 gallon heavily stocked koi pond system with a 1000
gallon vortex settling tank and two large trickle tower filters:
7-14 -02 received orp let orp probe set in pond for 48 hours to settle
down
7-16-02 6:30 pm 207 ORP reading
7-17-02 6: am 238 ORP reading, 4 PM 215 ORP reading
7-19-02 culled out 11 fish
7-24-02 4 pm 218 ORP reading
7-24 -02 cleaned settlement chamber 1000 gal water change in 8000 gallon
system.
7-25-02 reduced flow through submerged filter. Increased flow to in pond
return. Moved some plant stands to get better flow in pond.
7-26-02 6 am 360 ORP reading, 5 pm 315 ORP reading (this swing from morning
till night I think is caused by the difference in temp.) Started aeration
system running at night on a timer.
7-27-02 Rinsed off water hyacinths for fish to eat and dropped into pond
ORP dropped from the 300,s to 85 in less than 30 min??????? Took out hyacinths!
7-28-02 6 am 218
7-28-02 6 pm 267
7-29-02 6 am 331 High temps that day close to 100 deg. Ran aeration during
the day when we were not at home.
7-29-02 5 pm 328
7-30-02 5 pm 200 range??
8-1-02 Cleaned settlement chamber 2 PPM PP treatment with HP reversal.
8-2-02 5 Pm 238
8-4-02 7:30 am 380
This period of time I rebuilt my in pond TT and increased flow to 3000
gal per hour.
8-24--02 10:30 am 417
8-25-02 Cleaned settlement chamber PP treatment and reversed with sodium
thiosulfate orp 365
8-27 5:30 pm orp 377 temp78 deg
8-29-02 6:30 am 425
9-1-02 6PM orp 435 At this stage I am losing confidence in the probe as
the fluid has leaked out and there is a 2" long air bubble in probe.
Ordered new probe, received it a few days later.
After making adjustments to pond and management system pond has run in
the 300 range with the new probe, which checks to be correct from calibration
solutions and cross checks with other meters. The reading will drop to
the 280 range by day 7 after maintenance. That lets me know it is time
to clean out the filters.
In September a few leaves have started falling and just a few leaves will
drop orp into the lower to mid 200s – note from Conrad – I
observed the same rapid drop of ORP reading with leaf fall into the pond.
Comparable notes from Conrad’s ponds:
ORP can be stabilized in the 350 to 400 range without the low level PP
treatments under the following conditions:
1. When the mechanical filter system is maintained with a low level of
trapped solids,
2. With the use of activated carbon in the system for consistent DOC removal
3. With the use of massive trickle tower filters
4. With pond turnover rates at or above a turnover per hour. My turnover
rate on my outdoor koi pond is two to three turnovers through the filter
per hour, the turnover rate on my indoor koi pond is about once per hour.
Both run in the 350 to 400 ORP range when the pond is clean and the filters
are working properly, without adding potassium permanganate.
Massive leaf fall without quick removal will drop ORP below 300 quickly,
sometimes to below 200 if the leaves are left in the system very long.
Conrad uses the low level of PP treatment to increase ORP levels when
he does not have time to do a good mechanical filter cleanout when the
ORP level drops due to leaf fall or other temporary pond upsets. Conrad
also uses the low level PP treatment to prepare his pond for an actual
high level PP treatment for parasite control.
Use of ORP meters to monitor a high level PP treatment to kill parasites
A remarkable disadvantage of the use of PP to kill parasites is the uncertainty
of knowing the actual level of remaining active PP in the water to kill
the parasites. The problem is that the PP is frequently rapidly used up
by oxidation of DOC and solid crud in the pond system. An ORP meter allows
careful scientific control of killing parasites with active PP. The desirable
range of active PP is achieved when the ORP reading is in the 475 to 550
range for 2 to 3 hours by continued addition of PP to maintain the ORP
reading in that range. In a dirty pond this may require a total of up
to 6 to 8 PPM of PP powder, but in a very clean pond which has activated
carbon as a part of the filter system this may take as little as 1.5 PPM
PP charge.
Note 1: Do not allow the ORP reading to exceed 600; if it reaches that
level immediately reverse the treatment with either sodium thiosulfate
or hydrogen peroxide.
Note 2: Do not allow the fish to be subjected to water where the ORP reading
stays in the 550 to 600 range for more than 30 minutes without immediate
reversal with sodium thiosulfate or hydrogen peroxide, to allow longer
times at these levels is likely to kill fish by oxidizing the gills.
Note 3: The fish are safe for up to 4 hours at ORP readings in the 475
to 550 range. Longer hold periods may or may not harm the fish, especially
at the upper (525 to 550) end of the range.
Note 4: Do not run the high level of PP with ORP readings above 475 through
the biofilter since the useful bacteria in the biofilter can be killed.
Note 5: Take out any activated carbon for a high level PP treatment to
kill parasites.
ORP meters which have given me good service:
Cat No ORP3 at www.aquaticeco.com is on special currently at $99 and gives
good service for a battery driven unit.
Cat No. M35649 is an excellent plug in ORP meter which can be left on
line for readings anytime of the day or night. Also it is easy to adjust
calibration of this meter. Price $159
Cat No 7022 is required; it is a calibration solution at a 470 mV ORP
value. Price $16
Cat No CS is a cleaning solution made to clean the ORP probe if you believe
it may be fouled from service. Price $6.95 per pint.
Cat No SS is an electrode storage solution in case you need to store the
electrode out of the pond. Price $6.95 per pint.
Cat No SBT is a nicely designed bottle to store electrodes safely; if
they dry out, they are useless and can not be regenerated. Price about
$3 or $4 each.
ORP readings versus active PP levels
Since advising folks to use PP charges in the 0.1 PPM to 0.5 PPM range to help them manage their water quality, it occurs that folks may want to relate the ORP reading to actual live PP levels in the water.
I started this correlation with generic distilled water from the grocery store, Kroger's brand. To the distilled water, in one case, I added (by analysis) 220 PPM alkalinity, 40 PPM hardness, and 0.1% salt content to make the distilled water have near the properties of pond water, but without DOC (Dissolved Organic Carbon), or dechlor to "eat" the PP charge. This first experiment is labeled below as "pH 8.33" data. For the experiment labeled pH 7.4 below, the alkalinity was adjusted to 60 PPM, trace salt adjusted to 0.06%. For the experiment labeled pH 6 below, only salt at 0.06% was added to the distilled water - distilled water is always a bit acidic because of dissolved carbon dioxide content.
At these water parameters, at 25 degrees C or 77 degrees F, I collected the following data:
No PP dose gave 300 ORP at pH 8.33, 350 at pH 7.4, 370 at pH 6.0
0.2 PPM PP gave 360 ORP at pH 8.33, 420 at pH 7.4, 480 at pH 6.0
0.4 PPM PP gave 400 ORP at pH 8.33, 470 at pH 7.4, 560 at pH 6.0
0.6 PPM PP gave 420 ORP at pH 8.33
0.8 PPM PP gave 440 ORP at pH 8.33
1.0 PPM PP gave 470 ORP at pH 8.33, 495 at pH 7.4, 620 at pH 6.0
1.5 PPM PP gave 480 ORP at pH 8.33
2.0 PPM PP gave 510 ORP at pH 8.33, 510 at pH 7.4, 650 at pH 6.0
3.0 PPM PP gave 530 ORP
4.0 PPM PP gave 530 ORP at pH 8.33, 550 at pH 7.4, 680 at pH 6.0
5.0 PPM PP gave 540 ORP at pH 8.33
When chilled from 77 degrees F to 50 degrees F, the 5 PPM PP level at
pH 8.3 gave a reading of 580, up 40 mV.
Likewise, warming up from 77F to 103 degrees F dropped ORP reading by
20 units at pH 8.3.
