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Controlling pH in Biofloc Ponds


This long discussion (37 messages) took place on the Shrimp List, a mailing list for the shrimp farming industry.  The discussion got technical and sometimes strayed to other topics.  Here I concentrated on the responses that focused on pH.


Billy ( Dear All, I’m a shrimp farmer on the island of Bali in Indonesia.  I have a total of thirty ponds spread over two locations.  For the past two cycles, I’ve been using bioflocs in a minimal water exchange system.  Right now, I’m 60 days into the third cycle and the average weight of the animals is eight grams.


Growout Conditions:


• Total ammonia nitrogen = 0.5 parts per million and stable

• NO2 = 1 to 1.5 ppm

• NO3 = 10 to 50 ppm

• PO4 = 0.3 to 3.0 ppm

• Alkalinity = 100 to 140

• Flocs = 1 to 4 per milliliter

• Sun Light = very, very strong

• Feed = 30 to 32 % protein (the lowest protein % available)

• Carbon Source = wheat flour


My big problem is the diurnal shift in pH from 7.15–7.35 in the early morning to 8.0–8.5 in the late afternoon.  When this happens the shrimp start to show signs of stress.  When I increase the carbon supply, the morning pH drops even lower.


What’s causing the fluctuation in pH?


Alan ( Dear All, We grow our Artemia in a tank system and use a small amount of calcium carbonate as a substrate to maintain a constant pH of about 8.2.  In my backyard swimming pool, the pH is always “too high” and calcium carbonate forms on the tile.  It costs too much in acid to lower the pH because of the high calcium content of our water supply.  I am guessing that your water supply is mostly calcium deficient, so you might need to find a good source of calcium carbonate, like crushed coral, to buffer the water.


Ramon Macaraig ( Dear Billy, How big are your ponds and what are your salinities?  With those low pH readings in the morning, I bet your salinities are close to zero.


Billy ( Dear Ramon, The salinities in the biofloc pond range from 34-37 parts per thousand.  I don’t have enough fresh water to lower salinities.  It’s a concrete pond, about 2,500 square meters.  Over the last year a lot of small, oyster-like things have appeared on the pond walls after 45 days of growout.  When that happens, the alkalinity drops to 90-100, followed by even greater swings in pH.  I use as little water exchange as possible because other shrimp farms in the area have had problems with viruses.  I tried dolomite on a daily basis, but if I stop for more than three days, the alkalinity starts to drop again.  The oyster-like things on the walls seem to like the dolomite because whenever I use it, their populations increase.


My algae counts are less than 40–100 cells per milliliter.  After 90 days of culture, I begin removing some suspended solids.


At my other location, 50 kilometers away, the pH fluctuations are not as great because I can exchange more water and still maintain the right carbon/nitrogen ratio.


Job Madrona ( Dear Billy, Your water quality parameters are really not at optimum levels.  Is there any seepage?  What is the seepage rate?  What is your pond bottom substrate?  Do all your ponds have the same problem?


If you use molasses, instead of wheat flour, as your carbon source, it might help even out the swings in pH.  You can apply rice bran and molasses together.  Better yet, use probiotics to stabilize your system.


Billy ( Dear Job, There’s no seepage from my ponds.  After 40 days of culture, I add 10 centimeters of water weekly, but that’s only because I use the central drain once a day to remove sludge.


I’ve experimented with various amounts of molasses and found that it increased the range in pH, rather than decreasing it.


For the first 100 days of growout, I use sanitized seawater (sodium hypochlorite at 30 parts per million).  After the first 100 days, I have to begin exchanging some water to dilute the flocs because they become uncontrollably thick.  Then, I pump seawater directly without treatment until harvest at 120 to 135 days.


I add probiotics every other day in the hope that they will control some of the algae.


Increasing the amount of carbon has not helped with the swings in pH.


Last year, we were able to maintain the daily swing in pH to a half of a point.  It’s much worse this year.  It’s happening in all the ponds.  If it persists for ten days, the shrimp begin showing signs of stress.


We use 12-hp of paddlewheel aeration in each pond, stocked with 325,000 PLs.

I believe in the system, but I’m still learning by reading and doing.  I don’t have a formal education in the marine sciences.


Jim Anderson ( Billy, Based on what you have described, I would like to make a couple of suggestions.  The oyster-like things may be contributing to the problem by consuming alkalinity, which reduces your capacity to buffer pH.  The relationships between CO2, pH and alkalinity are at the core of your problem.  Your alkalinity has dropped.  Remember that as CO2 goes up, pH comes down as it utilizes alkalinity.  And floc systems produce lots of CO2 unless they have a lot of algae.  If alkalinity is high enough, then it buffers pH so there is little movement.  You mentioned the oyster-like things on your pond walls.  Oysters consume calcium carbonate (alkalinity) to produce their shells.  I don’t know if you have enough of them to make an impact, but it may be worth looking into.


Todd Blacher ( To me it seems that your pH fluctuations are the typical result of a heavy algal population.  You may have a much stronger bloom that you think.  The bloom is consuming CO2 during the day (pH rises) and producing CO2 in the evening (pH falls), which is the standard cycle and will have a direct impact on your day/night pH levels.


I personally do not feel that all the other factors that you mention are having much of an impact on your situation; I think the algae blooms are the basic cause of the fluctuations in pH.


I recommend that you try to kill off some of the bloom and see if it has the desired result.  You can use 50 kilograms per hectare of calcium hydroxide (CaOH) in the morning on a sunny day, when O2 levels are good—with your paddlewheels ON.  You need to be careful here because if you kill off too much of the bloom, you may have evening dissolved oxygen problems (which is why we usually perform this task in the morning with paddlewheels ON).


Dallas Weaver ( There are four ways to minimize diurnal pH variations:


1. Increase the carbonate/alkalinity ratio (a very nonlinear relationship) by adding very fine dolomite or very fine CaCO3.  Or add NaHCO3 or Na2CO3 (careful with this one as it can increase the pH to excessive levels before enough CO2 gets into the water to convert it to HCO3-).

2. Use deep ponds.

3. Decrease the ratio of photosynthesis to heterotrophic growth by increasing the C/N ratio.  Adding more carbon can create brown shade.

4. Add some shade.


Josh Wilkenfeld ( In 2008, our super-intensive raceways got way out of balance in terms of alkalinity and pH, and we were see-sawing to the point where we couldn’t keep a good balance between the two, even with the addition of sodium hydroxide or sodium bicarbonate.  In 2009, we were able to keep things under much better control with just the use of sodium bicarbonate.  Might this offer a solution to Billy’s problem?  Could he even out the alkalinity/carbonate problem by hanging bags of crushed oyster shell in front of some paddlewheels?


Jim Anderson ( All, It may be the algae, as Todd said.  In my first post to this discussion, I assumed that the algae was controlled.  It’s the first thing that causes daily swings in water quality chemistry.  Never ASSume.


Durwood Dugger( Josh and All, Any carbonate substrate you put in an aquaculture system will become insulated by biofilms very rapidly, which will slow up or effectively stop dissolution.  The solution (no pun intended) is to put the shell, aragonite, or other CaCO3 granular material into a hopper that can be constantly tumbled, so that it is self-abrading, self-cleaning.  A device designed along the lines of a small-scale pneumatic sand dredge works nicely—or better yet, a fluidized bed with an aragonite medium, which would insure maximum water/CaCO3 contact and dissolution.


Brian Boudreau ( Billy, Your parameters all look fine, except your alkalinity is a bit low.  Your pH swing 7.15 – 8.50 is about normal for pond culture and should not be stressing the shrimp.  A pH of 7.15 is well within range, even optimal, if you want to disfavor Vibrios that like pHs into the 8s.


Is there something else that could be causing the stress?


With a tank size of 2,500m2, a stocking density of 130/m2 and a relatively light feed load, it should be relatively easy to maintain alkalinity levels over the cycle with sodium bicarbonate.


In your response to Job, you mention draining 10 centimeters of water a week to remove sludge.  If you were to cycle your sludge (denitrification) until all the carbon and nitrogen were used up, you could produce a substantial amount of alkalinity—assuming that there was a fair amount of nitrate in the sludge.


Ramon Macaraig ( Dear Billy, The discussion has turned to algae as the possible culprit.  What’s your water transparency at 9:00 a.m.?  What are your dominant algae species?  What oxygen levels do you maintain in your ponds?


Billy ( Dear Ramon, Morning transparency is 25-30 centimeters (Secchi disk).  Evening dissolved oxygen is greater than 3.8 for all ponds, and paddlewheels are all on for 24 hours.  The dominant alga at this moment is Biddulphiophycidae, which is approximately five times larger than Chaetoceros.  There are probably 15 species of algae in our system.


Juan Aguirre ( Billy, What are the symptoms of your stressed shrimp?  Are you seeing mortality?  What is the color of your water?  What is the temperature in the morning and evening?  Vannamei or monodon?  You mentioned neighboring farms have viral problems.  What virus?  WSSV?


Roberto Marchesini ( Billy, When concrete tanks are not cured properly, pH can swing up and down without an apparent cause.  Concrete leaks chemicals that somehow affect pH.


Billy ( Dear Juan, I’m culturing vannamei.  I know it’s stress because the bodies of the shrimp turn white and a few dead shrimp float to the surface.  This year has been unusually hot and dry.


Pond Depth = 1.0 to 1.2 meters

Nighttime Dissolved Oxygen (9:00 p.m.) = 3.8 to 4.2 ppm

Afternoon Dissolved Oxygen (4:00 p.m.) = 7.0-8.5 ppm


Yesterday, I completed nine weeks of growout.  When I did my weekly sampling, the shrimp weighed between 8.3 and 10.0 grams.


Stress occurs whenever pH changes by more than a half of a point.


Josh Wilkenfeld ( Hi Dallas, We’ve been using sodium bicarbonate to control alkalinity in the super-intensive raceways here in Corpus, but I do wonder if that is a cost-effective approach for a large commercial system.


We usually try to keep our alkalinity in the range of 160-180.  I know that using sodium bicarb, if my pH is, for example, 7.4 and I want to raise the alkalinity from, say 105 up to 180 in a 40 m3 raceway, I need to add about 4.8 kilograms of sodium bicarbonate, which would also bring the pH up to about 7.8.  Can you give me an idea of how much dolomite or crushed oyster shell it might take to get the same result?  (As I’ve told you previously, I’m not a good chemist, so I have to admit that the sodium bicarbonate calculations are made easy for me using a program called AquaCalc by Dr. Nick Staresinic, an old friend from my early days in shrimp culture back in Galveston.)


Josh Wilkenfeld ( Hi Dug, Thanks—another bit of education—I had to look up “aragonite”.  If the quantity/m3 of water volume needed of whatever substance (aragonite, crushed oyster shell, or whatever) to maintain alkalinity and pH is reasonable, I’m sure that we can figure out a practical way to work it into our recirculating system.


Josh Wilkenfeld ( Dallas, Do you have a favorite pH sensor and controller system?  I’m setting up the automatic controls for CO2 going into our algae biomass raceways using individual pH probes and solenoid valves for each of the 12 units.  I’m looking for inexpensive equipment that is reliable and durable.


I’ve used several brands of pH controller—with a policy of never replacing a unit with the same brand.  I ended up with almost all Jenco products, their “3675” for indoor applications and “3679N” for outdoor applications.


For probes, I ended up with Innovative sensors M-10s.  They lasted at least one year.


Billy ( Thank you Dallas.  Yesterday I tried applying CaCO3 to see if it had any effect on pH.  What’s the best time of day to apply it?  What effect does it have on algae?


Dallas Weaver ( Adding CaCO3 is adding ground rock (limestone) with a very slow reaction rate and will only dissolve when the pH is less than 8.0, or so.  AM or PM is irrelevant with this mineral.  Many small additions are required.


Phosphate and Bioflocs

A Continuation of the pH Discussion


Th pH discussion got reignited a few weeks later when someone brought up the importance of controlling phosphate in intensive shrimp ponds, so I decided to add it here.


Adam Body (  Hi Billy [Billy is a shrimp farmer in Indonesia who started the original discussion on pH]: On our farm, we do not use a floc system, but we do use molasses and limited water exchange. We have found that a lot of the bloom variability in our ponds can be reduced by controlling phosphate. Over the last few months we have been trying a product called “Phoslock” and have found that we can take phosphate levels way down. We initially tried it to help control a Prymnesium parvum bloom (a toxin producing flagellated alga), and that worked well, so we are now using it as a general pond management tool.


Billy ( Thanks.  I’m doing that and things are getting under control.


Adam Body ( We did try other ways to control phosphate, but they were only temporary at best.  The Phoslock is permanent and safe.  This is a plug for an Aussie product—but it works for us.


Billy ( I’ll look for Phoslock in Indonesia.  One of my ponds has very high floc and phosphate levels.  The flocs block the sun, so there’s not much algae to absorb the phosphate.


I’m still doing a lot of observation, trying to figure out the what, when, how and why of bioflocs.  Sometimes it is really annoying because what happens seems to be just the opposite of what’s supposed to happen—and that’s scary.


Alain Herni ( My experience with flocs was in the 1970s with Harvey Persyn and the Ralston Purina team in Crystal River, Florida, USA.  We can debate the stability of floc systems forever, but in my view, they are inherently chaotic, very sensitive to initial conditions like water chemistry and light intensity.  Site variables—soils, air and weather—also play a role in the behavior of these systems.  In addition, many of the organisms in floc systems, like rotifers and ciliates, experience chaotic population cycles that contribute to instability.


And, yes, phosphates tend to build up in biofloc systems.  Shrimp feed, for example, rapidly increases the amount of phosphate in the water.


The IIFREMER AQUACOP team in Tahiti runs biofloc experiments in 1,000 m3 tanks using Penaeus vannamei.  We implement our biofloc technology at Sopomer, a small commercial farm in Tahiti that has ten 1,000 m3 tanks and was able to produce 20 metric tons per crop in a floc system with AIRE-O2 aerators.  The main problem was avoiding green filamentous algae.  We also use floc systems in small tanks of 10 m3 to maintain our P. vannamei broodstock.  We have more than 20 generations of P. vannamei derived from a small population of postlarvae introduced in the mid 1970s.  In aerated small tanks, you can easily control the light and other variables, so they are more stable.


Adam Body ( I have very little experience with floc systems.  Our system uses minimal water change and organic carbon (molasses) to balance the excess nitrogen originating from the protein in the feed.  Alain’s comments regarding chaotic systems are quite correct.  Our “models” cannot possibly provide us with a forecast of what will happen in outdoor ponds from day to day.  What we have found, however, is that by reducing the supply of phosphate we can limit the intensity of the chaos.  They are still chaotic, but the numbers are reduced because there are fewer primary nutrients, like phosphate, in the water, so the algal biomass is limited, allowing more control over water transparency and subsequently limiting the size of diurnal pH and oxygen fluctuations and allowing development of more stable and diverse blooms.


Billy ( Yup, that’s true.  How high does phosphate get before you begin controlling it?  1 ppm?  0.5 ppm?


At this moment, my phosphate levels are less than 2 ppm, except for one pond where the floc is dense and the phosphate high.


Adam Body ( Once the bloom is around 35-40 cm transparency we aim to cut phosphate down to less than 0.03 ppm.  Any higher than that and the bloom starts to grow and your Secchi disk reading will drop to 30 cm or less.  I asked our feed manufacturer about the amount of phosphate in our feed, and he said it was around 17 kg per ton, with about 11 kg per ton of it as available phosphate.  That’s a lot of phosphate.  Our measurements indicate that a lot of it must be in another form because we don’t see anywhere near that much coming from the feed that we use.




The Process of Removing Phosphate


This is the third installment of a discussion from the Shrimp List, a mailing list for the shrimp farming industry.  The first installment covered managing pH in biofloc ponds.  The second shifted the discussion to managing phosphates in biofloc ponds.  That discussion continues here:


Adam Body ( Some weeks ago we had a discussion on the Shrimp List about phosphate management in shrimp ponds.  I have collected some information on the topic and posted it to our website.  It includes some of the things that we are finding out about our ponds and the way the algal bloom reacts when we take out free phosphate.  You can check out that information at new tool for pond managers.html.


I’m very keen to hear from anyone who may have some experience or knowledge about  “alkaline phosphatase” because I think it is playing a role in the pond, but I really don’t know....


A Little Help from Wikipedia: Alkaline phosphatase is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules, including nucleotides, proteins and alkaloids.  The process of removing the phosphate is called dephosphorylation.  As the name suggests, alkaline phosphatases are most effective in an alkaline environment.


Alan ( I would also be interested in such a discussion.  I recently patented a process for reducing phosphates in wastewater by culturing a phosphate reducing floc in an aquaculture system.  My research has shown that the phosphate reducing floc can be cultured using standard practices.


One of the references on your webpage suggests a phosphate threshold of 0.03 ppm (mg/liter).  I think that might be a typo.  I have found the threshold to be closer to 3.0 ppm.  I don’t even have a phosphate kit that would measure the small ranges mentioned on your web page.  One of your references says:  “Practice suggests that pond PO4 levels should be kept around 0.05 - 0.2 ppm until the Secchi transparency reaches about 45 cm.”  My kit only measures phosphate from 0.25 to 10 ppm.


Any comments?


Dallas Weaver ( Interesting that you got a patent on it (number and country?).  For decades, the sewage treatment industry has been using bioflocs to remove excess phosphate under aerobic conditions.  Then, they feed it back up under anaerobic conditions.  It seems that the old concept of  “obviousness to practitioners of the art” is no longer relevant in patent law.


If you want to use phosphate limitation to control algae species or growth rates, you are talking about a lot less than a 3-ppm (parts per million) threshold.  That 3-ppm level will grow enough algae to crash a pond on low DO.  I don’t know the correct level, but I do know it is much less than 3 ppm.


Alan ( Interesting.  Since my application and usage are unlit tanks under intensive aeration, the crash did not occur, and I was showing phosphate above 5 ppm without a crash.  The critters just didn’t grow at these high levels, and ammonia appeared to be burned without additional carbs; that is, the ammonia wasn’t available to create proteins because it became nitrogen gas.  Again, I don’t have specific proof, but I saw small bubbles that appeared to be nitrogen on the tank surface.


The United States Patent Office issued the patent, Number 7,736,509.


I have found that phosphate is a necessary component to feeding my shrimp (creating lipids).  I hope that is no big surprise.


Adam Body ( Alan, I use the Palintest photometer and the low range (LR) set of reagents which are very capable of measuring the low ranges mentioned on our webpage.  So, no, the figure is not a typo.  Only under 0.1 mg/l PO4 do I see any significant changes, and under 0.05 mg/l PO4, it can really make a difference to the pond bloom.  I am measuring free PO4 with the sample filtered through a 1-micron filter, and the numbers all relate to mg PO4 per liter.  As far as generalities about PO4, I would be extremely reluctant to transfer these figures to other farms or operations because all farms are different (soil types, feed inputs, water parameters), but I can state that having the ability to selectively take out PO4 simply and cheaply gives managers a degree of control over their algal blooms that they have not previously had.  How they apply this new management tool is up to them, but a good first step would be to begin monitoring your PO4 to see if its levels are affecting your algal bloom.


Alan ( I control tank blooms by recording the amounts of carbs, nitrates and phosphates in my system.  When one of them gets out of balance, I change the percentage of it in my feed.  My worst mistake was adding too many carbs and getting fungal blooms.  They seemed to affect gill function and cause disease.  By limiting the carbs and without a strong light source, the phosphates seem to have no possibility of creating an undesirable bloom, which might explain why my system does not crash with such high phosphate levels.  It also says something about the differences between indoor tank culture and outdoor pond culture.  I ran out of test reagents last week and just received a new shipment.  I am currently working on dropping my PO4 level from 5 mg/liter!  The shrimp are OK.


Phosphate levels change from day to day.  Check them on a clear day, then on a cloudy day.


Adam, I am almost certain that your ponds could biologically  “sink phosphate laden wastewater” by the biological processes outlined in my patent.  If you can manage a pond at those precisions, I could probably throw a very high phosphate laden wastewater in your ponds, and it would sump out most of the phosphate before your test kit could measure a change.


Robinson Bazurto ( Using an algal turf scrubber, you are able to remove huge amounts of TP (total phosphorus) and TN (total nitrogen) and reduce TSS (total dissolved solids).  In addition, an algal turf scrubber increases DO levels, requires less labor and provides algae, which can be used as a feed.


Adam Body ( Alan, thanks for the comments.  As you indicated, there is a very big difference between tanks and large outdoor ponds.  This probably accounts for our different experiences.


Alan ( Adam, thank you, too.  I am keeping one outdoor tank, and as if on cue, it experienced a bloom, which stressed the shrimp.  I think I dodged a bullet because I wasn’t hitting them with too much phosphate.


Eric De Muylder ( I am surprised that nobody talks about avoiding high phosphates in the water in the first place.  That should to be the focus.  One of the reasons for high phosphate levels in shrimp pond water is the utilization of low available phosphate sources like monocalcium phosphate and dicalcium phosphate in shrimp feeds.  It has been proven over and over that they are not available to the shrimp, but most South American and Asian shrimp feed manufacturers are using them.  Removing them from shrimp feed, which would not affect shrimp growth, should be the first step in reducing phosphates in pond water (and effluents!).  The next step should be adding digestible sources of phosphates to shrimp feeds.  The problem here is that some of those sources are highly water soluble and leach out of the feed pellet before the shrimp consume them.


David Griffith ( Eric—of course, of course—that’s the first thing we looked at and indeed are still discussing it with our feed supplier.  We don’t use monocalcium phosphate or dicalcium phosphate in our shrimp feeds.


What has been an important issue however is our elevated FCRs (feed conversion ratios) caused by low survivals, so indirectly the feed, whatever its source of PO4, is the major cause of the problem.  With better survival rates and consequent improvements in FCRs, the problem will be reduced, no doubt, but not resolved.


Sources: 1. The Shrimp List (a mailing list for shrimp farmers).  Subjects: Controlling pH Range (February 28 to March 31, 2010); Floc System and Bloom Variability (May 7, 2010); and Phosphate Management (August 13–25, 2010).  2. Bob Rosenberry, Shrimp News International.  Update.  March 22, 2017.


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