Crystal clear water is often indicative of a well-balanced ecology in mature pond systems. It allows sunlight to penetrate into all layers of the water column. This supports a community of primary producers that rely on light energy to convert available nutrients into organic matter, which becomes a vital source of food for higher forms of life.
Apart from its functional benefits, clear water is also visually stunning and allows us to appreciate the living contents of a pond. It facilitates observation of the ecological connections that allow them to survive and reproduce. Unfortunately, many common situations can destroy this almost romantic image of a clear pond. Sediment disturbance (especially of clay) and erosion due to heavy rainfall can drastically increase mineral turbidity levels.
When pond water is fully turbid, submerged plants can die off, fish gills can become damaged, and beneficial plankton communities can perish. It is important to address turbid water in a timely manner to prevent irreversible damages to pond life. One of the most customary measures of reducing turbidity is with the use of chemical additives, such as gypsum. This article will take you through its chemistry, proper application, and mode of action.
What Exactly Is Gypsum and How Does It Work?
Gypsum, or calcium sulfate (CaSO4), is a natural, water-soluble mineral that often occurs in sedimentary rock systems. Considered the most common of sulfate minerals, it is deposited by saltwater, lakes, volcanic vapor, and hot springs. It forms when sulfuric acid comes into contact with calcium carbonate. Gypsum is now commercially mined from quarries all over the world. It can also be synthetically produced in many types of processing plants.
Today, gypsum is a primary component of blackboard chalk, drywall, and plaster. Its application extends into the aquaculture industry, where it is often considered a type of lime. As a pH-neutral chemical, it is an ideal additive that can be used without the risk of drastically altering pH levels in both soil and water. Due to its chemical composition, it can aid in reducing turbidity in ponds by forcing clay particles to sink.
In earthen ponds, negatively-charged clay particles remain suspended in the water column because they repel each other. On their own, single clay particles are so fine and light that they are unable to settle, even in still water. When gypsum is introduced into the water, it serves as a coagulant that forms bridges in between the negatively-charged clay particles. This way, they can begin to aggregate and form “floccules” or “flocs”. Once the aggregations are large enough, they can sink to the pond bottom and remain there.
When to Use Gypsum
As gypsum is a calcium compound, it should be used only in ponds with a low water hardness (below 37.5 ppm). If your pond water’s hardness exceeds 50 ppm, gypsum may not be effective at reducing turbidity. Moreover, it is more successful in fairly alkaline ponds (>50 ppm) because its sulfate component should supposedly bring down alkalinity to be at par with hardness.
When confronted with a turbid pond, you’ll also need to contemplate the source of the suspended material/s. If it is of a mineral or sediment source, gypsum may be effective as a chemical additive. In contrast, if the turbidity is caused by a plankton bloom, the coagulant action of gypsum would be ineffective at reducing microbial density. It can technically reduce the chances of a plankton bloom by increasing hardness, precipitating phosphates, and promoting water clarity, but it shouldn’t be used as a solution to existing blooms.
Gypsum should only be applied to pond water once the source of suspended solids has been determined and addressed. Otherwise, its repeated use can become costly and its effectiveness may be deemed short-lived or very poor. If water hardness and alkalinity parameters don’t match those indicated above, consider using alternative chemical amendments that are more suited to a lower alkalinity or high hardness.
Types of Gypsum
Depending on the source and the extent of processing, there are several types of gypsum. Some of these are more suited for use as a type of soil fertilizer rather than as a pond water additive. The type/particle size you use will also determine how to most effectively spread the material across your pond. Note that the application rates for each of these would vary depending on the properties of your pond water and its level of turbidity.
- Ultra-fine gypsum – This type can be used as a water clarifier, a compost additive, and as a soil conditioner for agricultural crops and lawns. Its particle size is similar to that of dust – less than or equal to 1.5 mm. As it is so fine, it is supposedly fast-acting and can be hand-applied.
- Granular gypsum – This type generates less dust compared to ultra-fine gypsum. Its uses are similar, but it should ideally be spread with a fertilizer spreader (the land-based equivalent of a motorboat) or allowed to dissolve in clean water before application as a slurry. Particles can be as large as 3 mm.
- Pulverized gypsum – This type has highly variable particle sizes, resulting in extended-release.
- Pelleted gypsum – This generates the least amount of dust and also treats water through slow release. Particles range from 3 – 6 mm and are less likely to generate a uniform slurry.
Costs and Effectiveness of Gypsum
The key to the effectiveness of gypsum is its proper application, given appropriate conditions. It is considered effective about 65% percent of the time in sufficiently alkaline water. Compared to other amendments, this rate of effectiveness might be considered weak.
An effective gypsum treatment should reduce turbidity within a few days and have long-lasting results. However, natural causes (e.g. floods, heavy rain, nuisance fish) may resuspend or introduce more floating sediments. Without proper land management or if the sources of sediment are not addressed, gypsum may appear ineffective.
In terms of cost, gypsum is considered an average-priced solution (agricultural lime is cheaper) to water turbidity. Repeated usage would not be cost-effective, especially as the effects of gypsum application are supposed to be long-term. For small ponds that would require minimal usage of any type of amendment, the cost of gypsum could be considered negligible. It would follow that large aquaculture ponds, requiring the use of tons of gypsum each year, would incur significant costs. A ton of gypsum can cost more than 150 USD, depending on how processed or clean the material is. A 50-pound sack of ultra-fine gypsum would cost around 10 USD.
Is Gypsum Safe to Use in Fish Ponds?
Gypsum is generally safe to use in fish ponds as long as it is applied in adequate doses. It should also be distributed throughout the pond, and not simply dumped into a single area. Pond water that is left turbid or muddy for long periods of time can be more detrimental to its occupants. Some types of fish and crustaceans may tolerate or even thrive in a turbid system, but others (such as most ornamental fish) will suffer in turbid water.
Applying gypsum in slurry form instead of as a powder will help prevent it from being accumulated in the organ systems of fish. Try to look for lab-grade or processed gypsum with a higher purity rate, as crude gypsum may be contaminated with other types of raw materials.
How to Apply Gypsum and Determine Dosage
Once you’ve determined that the suspension of clay sediments is the primary cause of turbidity, check your pond’s hardness and alkalinity. If these meet the general requirements for gypsum use (hardness <37.5 ppm, alkalinity >50 ppm), you should conduct a bucket or glass jar test. This will help you determine the appropriate dosage of gypsum for your pond’s level of turbidity. It is also important to obtain measurements of your pond’s volume as a function of its surface area and depth.
Accurately measure the amount of water that goes into each bucket or jar. Each container should have the same amount of water, which you must collect from an ideal section of the water column. Avoid disturbing the pond bottom when collecting water. To make things simple, try to have 1-gallon of water each in at least 5 transparent containers. Set one container aside and label it – this will serve as your control.
For the remaining 4 – 5 containers, test various concentrations of a standard gypsum slurry (made by mixing 2 tablespoons of gypsum into a gallon of clear pond water). Label containers according to the amount of slurry that is added to each (for example, Jar 1 for 1 tbsp slurry added to a gallon of the turbid pond water, Jar 2 for 2 tbsp slurry, and so forth). Thoroughly stir the mixtures and leave them undisturbed for 12 hours.
Pinpoint the lowest possible concentration (least number of tbsp of gypsum slurry) that managed to clear the pond water, and consult the table below for the proper dosage. Make sure to use accurate conversions when calculating dosages for smaller pond volumes. Pre-mix the corresponding amount of gypsum with pond water and distribute it as evenly as possible around the pond.
| # of tablespoons of GYPSUM SLURRY per 1-gallon jar | Corresponding amount of gypsum to apply to pond water (in lbs/acre-foot) |
| 1 | 80 |
| 2 | 160 |
| 3 | 240 |
| 4 | 320 |
| 5 | 400 |
Alternatives to Gypsum
If your pond’s water chemistry doesn’t meet the requirements for gypsum use, consider using other chemical coagulants to reduce turbidity levels. These include agricultural lime (calcium carbonate), Epsom salt (magnesium sulfate), alum (aluminum sulfate), and synthetic polymers. Use of these amendments will require a bucket test prior to their application as well. You can opt to test all of these simultaneously so that you can compare results.
Hydrated lime (calcium hydroxide) and burnt lime (calcium oxide) should only be considered as alternatives to gypsum if the pond does not contain fish. Hay may be used as an organic additive, but note that it can increase the acidity of water and deplete oxygen levels.
As always, taking extra care to prevent increased turbidity will be the most effective means of maintaining clear water. Using pond liner in areas with a high clay content would remove the issue of sediment suspension from within the pond itself. To avert the entry of sediment from run-off or via pond border erosion, it would help to maintain heavily planted edges.
Other preventive methods include fencing off the pond to thwart the entry of livestock, avoidance of stocking bottom-dwelling fish, and using surface instead of bottom aerators. Turbidity is a common occurrence in new outdoor water systems, so a little patience is crucial at the start. Conservative trial and error, coupled with careful observation and frequent water testing, should help you determine the best treatment for your pond.
