Water Quality – Ponds

They're nothing like aquariums.

It might seem that ponds are just very large aquariums, and therefore reading the article on water quality for freshwater tanks in this issue would be sufficient. The problem is, while pondkeepers measure many of the same water quality parameters, the conditions related to a body of water outdoors in the ground have effects on water quality that aquarists do not have to contend with.

Regular readers of AFI know that water quality is something of a preoccupation with its feature writers and columnists. And with good reason. Nothing influences the health and the well-being of aquatic animals as much as water quality. The finest quality koi given premium quality food will languish if kept in substandard conditions. Conversely, otherwise “pool quality” kept in very high quality waters can grow to be magnificent animals.

The explanation for this is simple: fish are truly products of their environment. Unlike land-dwelling animals, fish metabolism and biological functions are directly linked to the physical and chemical properties of the medium that surrounds them — water. Moreover, pond water constantly floods a fish’s tissues, mixing with body fluids. Thus, poor or inappropriate water quality translates directly to poor fish health.

What Matters?
So what defines good water quality in ponds? There are potentially dozens of physical and chemical characteristics that we might consider worthy of measuring, monitoring and controlling. My experiences suggest, however, that the conscientious pondkeeper need only be concerned with a handful.

Physical characteristics:

  • temperature
  • flow rate

    Chemical characteristics:

  • pH
  • dissolved oxygen
  • ammonia
  • nitrite
  • animal-derived dissolved
  • organic carbon (DOC)

    Biological characteristics:

  • bacterial load
  • zooplankton and
  • invertebrate load
  • fish load
  • plant load

    If these characteristics are properly regulated the rest will fall naturally into place.

    Static vs. Dynamic Characteristics
    Invariably, pond hobbyists want me to tell them the optimal pH, temperature, flow rate and related values they should strive for in their ponds. Life would certainly be much easier for us all if good water quality was defined by a fixed set of water quality parameters. But it is just not that simple.

    Ponds are highly dynamic aquatic environments. After all, ponds exist at the mercy of the elements — wind, rain, sunshine, soils and vegetation all exert great influences on water characteristics. Many of these characteristics fluctuate on a daily, monthly and seasonal basis. Consequently, pond fish and plants are, of necessity, fairly adaptable. They are used to change.

    Consider, for instance, how water temperature, pH and dissolved oxygen move together as part of a daily cycle. The morning sun warms the air and ground, with some heat transferred to the pond. Sunlight falling directly on the pond heats the water further.

    The brightening morning sky also stimulates aquatic plant photosynthesis. The plants begin to remove carbon dioxide from the water and produce oxygen. Removing CO2 increases pond pH, an effect that is especially noticeable in ponds with heavy algae loads.

    As the sun sets and the air and ground cool, so does the pond water. Evening also sees photosynthesis subside and the switch to plant respiration dumps carbon dioxide into the water. This sends the pH down.

    The magnitude of daily fluctuations in pond water quality is influenced by many factors. A fully exposed ornamental pond may experience temperature changes in excess of 10 degrees Fahrenheit over 24 hours. (Much larger natural ponds do not have such large temperature fluctuations because of the substantially greater volume of water they hold.) The pH might shift by as much as a full point or more between early morning and late afternoon. Dissolved oxygen levels may rise and fall by several parts per million (ppm) — particularly if the water flow rate is low.

    Monthly and seasonal changes follow a more familiar pattern. We expect pond temperatures to be colder in winter and warmer in summer. But you may not realize that many other physical, chemical and biological attributes change across the seasons as well.

    Obviously it makes little sense to talk about static water quality characteristics. What we need to do is talk about ranges.

    Physical Characteristics
    Water temperature is very important because fish are ectotherms. Unlike mammals and birds, fish cannot generate heat. When rabbits huddle together they can keep each other warm by sharing internally generated body heat. For fish, their body temperatures are determined by the temperature of the water in which they live. The continuous influx of water flooding their bodies keeps them in thermal equilibrium with their surroundings.

    In all animals vital metabolic processes, such as digestion and immune response, are very sensitive to body temperature. Because water temperature is the determinant of fish body temperature, it plays a correspondingly significant role in determining the health of the fish.

    During the seasons of spring, summer and fall, water temperatures between 60 and 78 degrees Fahrenheit are comfortable for commonly kept pond fish (pondkeepers attempting to raise trout would want temperatures in the 55 to 65 degree range, and those with tropical ponds would want 72 to 82 degrees). In winter, water temperatures below the mid-40s tend to stress koi and single-tailed goldfish. Fancy goldfish appear to suffer somewhat when temperatures drop below the mid-50s for more than a week or so.

    If solar heating and evening cooling produce wide temperature variations each day, there are several remedies. Tree cover over and around the pond can moderate the effects of solar heating. Shrubs that surround the pond can reduce the evaporative cooling produced by wind blowing across the pond surface. Extensive numbers of floating leaf plant, such as lotus and waterlilies, also reduce solar heating effects, as well as evening cooling. In areas of the southwest where tree cover is difficult to establish, or where trees do not fit the landscape ornamental canopies serve to shade ponds very effectively.

    The flow rate in the typical ornamental pond refers to the recirculation rate of the water. That is, how long does it take for an entire pond volume to be circulated? Too low a flow rate can leave the pond at the mercy of many influences that cause pond pH, temperature, dissolved oxygen and other physical and chemical properties to change significantly — and often beyond the safe limits of fish tolerance.

    Conversely, too high a flow rate can stress some fish — especially fancy varieties of goldfish — as they fight to maintain positions in the current. Very high flow rates also increase turbidity in ponds by preventing suspended particulates from settling out.

    A well known rule of thumb is to set flow rates between one-half and one pond volume per hour. I recommend using this guideline.

    Some may be surprised that I omitted turbidity from my list of important water quality characteristics. Turbidity refers to the amount of suspended matter — dirt, organic particulates, plankton, etc. — in the water. Turbidity determines visibility in the pond (how far down into the pond you can see your fish).

    Pondkeepers are inordinately concerned with this physical characteristic, believing that pond water should be as particulate-free as aquarium water. This is really not possible, and is certainly not necessary from the fish’s perspective.

    With the exception of ponds truly laden with silt or fish wastes, turbidity has little demonstrable effect on pond fish. Carp, after all, create turbid waters by rooting around pond bottoms.

    Whether turbidity indicates a real problem depends on the type of particulate matter suspended in the water. Some of the best koi-rearing ponds are turbid throughout the summer. But the materials are clay silts, phytoplankton and zooplankton.

    What should be of concern is turbidity resulting from very high fish waste and food waste loads, which are the causes of serious infectious disease problems. This waste matter serves as a breeding ground for pathogenic bacteria and viruses. If it is “thick” enough to cloud your water, this indicates very dangerous conditions. This material can also rob pond water of oxygen.

    High suspended fish waste loads are also a sure sign that there is too great a fish load in the pond, which adds significantly to the likelihood of disease outbreaks, oxygen depletion and so on.

    When the suspended matter is largely vegetative in nature, such as planktonic algae or decomposing leaf matter, it is rarely a reason for concern. It may be ugly, but it is not threatening unless it is so thick that it clogs the fish’s gills. Usually a minor improvement to mechanical filtering, such as incorporating a settling basin into the pond design, and reducing the flow rate and pattern in the pond will allow these materials to settle out.

    Turbidity produced by planktonic algae can be minimized by controlling nutrient levels, carbon dioxide concentrations and solar energy in the pond. This is discussed below.

    Water color is also a preoccupation of pond hobbyists that is largely an aesthetic, not a fish health, issue. Green water is due to planktonic algae. Tea-colored water is the result of tannins leaching from decomposing leaves in the pond. Unless leaves are removed promptly this coloring is unavoidable. Brown-colored water has several possible causes — dead and dying planktonic algae produces a brown coloration that disappears once the material settles out, and suspended clay or peat silts can also produce brown colors.

    To be sure, green or brown water offends many people. It is of no significance to aquatic animals. Carbon filtration can remove tannin coloration and some green coloring from plant materials, but I find this a wasted effort. It works for only very short periods of time, and very large amounts of carbon are required to effectively reduce color. All of my ponds have a nice transparent green tint, indicative of biologically healthy pond water.

    Chemical Characteristics
    Overall, the chemical characteristics of pond water have a greater effect on koi and goldfish than do water’s physical characteristics. For this reason, I suggest that this is where pondkeepers put their efforts.

    pH refers to the relative concentration of acids in the water. A value of 7.0 is considered neutral — less than 7.0 is acidic and more than 7.0 is alkaline. Fish metabolic functions are pH sensitive and, although most pond fish will tolerate a range of pH conditions, extremes of acidity or alkalinity must be avoided.

    I recommend that the pond pH be kept between 6.8 and 8.0. The lower limit (6.8) should be an early morning low, and the upper limit (8.0) should be a late afternoon high. But do not get too obsessive about this. If natural water conditions yield a pH of 6.5 to 8.3, leave it alone. Fooling with pH puts your fish in danger.

    Observation suggests that goldfish and koi colors appear a bit more intense at pH ranges between 6.5 and 7.0. This is most likely an optical effect — the fish’s slime coat thins at lower pH values. But before you start fiddling with the pH to get “better” colors you should know that a thinner slime coat means substantially poorer defenses against external parasites and bacteria. I prefer healthy fish with slightly muted colors to marginally more colorful diseased fish — and so should you.

    Natural biological processes in the pond will tend to drive the pH down over time. I recommend pH testing on a weekly basis. If the pH seems to be drifting below 6.8, I suggest you increase the amount and frequency of water changes. This is the most subtle way to stabilize water quality. Baking soda, oyster shells or agricultural lime can also be used in limited amounts, but great caution is advised.

    You might also consider reducing the fish load. Rapid declines in pH levels often indicate the fish waste load (including urine) is too great for the pond. The build up of organic acids and high rates of nitrification (which do acidify the water) can rapidly overwhelm the natural buffering capacity of the pond water.

    Dissolved oxygen is another critical element of pond water quality. The concentration of dissolved oxygen in the water determines how much oxygen is available to support fish respiration and how easy it is for fish to extract that essential gas from the water.

    Fortunately, goldfish and koi are fairly efficient oxygen extractors and users. Nevertheless, you should strive to keep dissolved oxygen concentration above 6 ppm. The larger the fish the greater their sensitivity to low oxygen levels. In other words, a pond with several large koi might experience deaths, whereas a similar sized pond with many small goldfish might escape catastrophe.

    There is no excuse for failing to maintain adequate dissolved oxygen concentrations. This is easily accomplished by maintaining a modest fish load and using artificial streams and waterfalls (or bubblers and venturi jets) to aerate the water.

    As I noted earlier a heavy fish loads places an increased burden on pond dissolved oxygen levels. First, fish respire using up oxygen in the water. A few large koi or a quantity of smaller fish can place a very large oxygen demand on the typical garden pond.

    Second, the nitrifying bacteria in the pond (and the biological filter if there is one) use copious quantities of oxygen to convert fish-toxic ammonia to harmless nitrate. Those little bacteria are fierce competitors for dissolved oxygen, much to the fish’s detriment. But even the nitrifying bacteria are affected if dissolved oxygen levels drop too low. They slow their detoxification efforts — resulting in an ammonia buildup.

    Indeed, ammonia and nitrite should be considered poisons in the pond environment. During most of the year these two nitrogen compounds should not be present in measurable quantities. The exception is in spring in cold climates where pond biology (nitrification) must restart from scratch. Then you are likely to get ammonia and nitrite readings for weeks. Yet even in this instance the fish load should be low enough to hold temporary ammonia and nitrite spikes well below dangerous levels.

    The single greatest source of ammonia in ornamental ponds is fish. The next is overfeeding. Put the two together — heavy fish loads and over feeding — and your pond is a disaster waiting to happen.

    Fish can tolerate higher concentrations of total ammonia at lower pH levels. Therefore, ponds with seasonal ammonia spikes might best be kept with pH values 6.8 to 7.0. Some pond keepers use ammonia removers or zeolites to lower ammonia levels. Although these products do work, they are expensive and imprecise. Fish load management is the best strategy.

    Nitrite is in intermediate product in the nitrification process. If pond biology is in balance, nitrite levels should not register using hobbyist test kits. Nitrite is more toxic than ammonia, and any reading should be cause for concern.

    Temporary nitrite spikes can be tolerated by fish with the addition of 0.1 ppm of salt to the pond water. This light salting will not harm plants. (I should note here that maintaining permanent low salt concentrations in ponds is a bad idea. It does not have any lasting impact on bacterial or parasite counts. It does not really help the fish. Salt should be considered a temporary water treatment in freshwater ponds.)

    The last chemical characteristic I will mention is dissolved organic carbons (DOC). DOC refers to a host of carbon-based substances that exist in the pond water.

    But the only types of DOC that should concern pondkeepers are those derived from animal sources — metabolic and biological byproducts, such as fats, proteins, amino acids and so on. These substances have an insidious impact on fish health and are directly linked with the presence of disease-causing bacteria in pond water. Ornamental ponds with dense fish populations always have unusually high animal-based DOC concentrations, whereas in natural ponds the overwhelming proportion of DOC in the water is plant-derived and has no impact on fish health.

    The only proven way to control DOC levels is to maintain low fish loads and carryout frequent and substantial water changes. Weekly water changes of 10 to 20 percent in moderately stocked ponds are advisable.

    A pond foam fractionator will also remove DOC (as well as algae and bacteria suspended in the water), but we do not know its relative effectiveness nor the types of DOC it removes best. In regions where water restrictions limit water changes, foam fractionators in combination with fish load control are a fine alternative.

    Biological Characteristics
    Much of what I have to say in this section has been already been alluded to. Nevertheless, the key points are important enough to highlight again.

    The bacterial load in ornamental ponds is a very important aspect of water quality. On the one hand, we want high concentrations of nitrifying bacteria to detoxify ammonia. We also want large numbers of vegetative decomposers to breakdown dead plant material that falls into the water. These microbial assistants are essential to maintaining good water quality, and without them our ponds could not support much life.

    On the other hand, we want very low concentrations of protein-consuming bacteria, such as Aeromonas and Pseudomonas. These bacteria live on animal matter and when their numbers get out of control they can pose disease serious threats to animals.

    Realistically, hobbyists have no way of measuring or monitoring bacterial loads in their ponds, let alone a way to distinguish desirable from undesirable bacteria species. We can, however, infer bacterial loads from other characteristics.

    The main indicators are fish load, fish waste load and food waste load in the pond. When the fish load and the fish waste load are high, both the total number of bacterial cells and the bacterial density (number of cells per unit volume of water) will be high. More bacteria mean a greater burden on fish immune systems. Higher bacterial densities also mean greater chances of any given fish becoming a host. Conversely, high fish loads (densities) mean a greater chance that any given pathogenic bacterium will find its way to a host.

    My advice on fish loads is well-known to AFI readers. Keep the load well below 1 kilogram of fish per 1000 liters of water. This is roughly two 12-inch koi per 300 gallons of water for ponds with effective biological filters.

    If you want to put a lot of something in your pond, make it plants. In natural ponds we find that the ratio of plant mass to animal mass is approximately 10:1. This is a good model for ornamental ponds. Trying to reproduce this ratio will help you keep the fish load down, if for no other reason than you will be spending all your money on plants.

    Quite seriously, planting 10 pounds of aquatic plants for every pound of fish will create a very healthy pond environment for your fish, and treat you to the true beauty of pond keeping. In particular, heavily planted ponds exhibit much more stable water conditions. Temperature, pH, dissolved gases and so on still fluctuate, but within narrower limits compared to ponds where fish dominate plants. By any measure water quality in heavily planted ponds always surpasses the water quality in fish-dominated ponds.

    And believe it or not, one of the most helpful plants in the pond is algae. In particular, algae are ravenous consumers of ammonia. Spring algal blooms protect your fish by keeping pond ammonia levels in check when nitrifying bacteria are not yet doing their job. (Indirectly, therefore, algae also prevent nitrite poisoning.) Thus, green water in spring should be seen as an indicator of too high a fish load for existing conditions. But nothing should be done to kill the algae unless the source of ammonia production is removed first.

    Yes, I have said too much algae can cause pH and dissolved oxygen problems. These effects, however, can be easily dealt with via good aeration and pond water recirculation. Send water over a cascading waterfall, through an artificial stream or through a bubbler and all will be fine.

    Finally, I will mention zooplankton and aquatic invertebrates. In my view, these are important biological indicators and contributors to pond water quality.

    Although we associate worms, snails, insect larvae, planaria, Daphnia and so on with “dirty” water, most of these animals are pretty finicky about water quality. (Yes, some do indeed prefer polluted waters, but these are the exceptions.) If nature has found your pond a good place to proliferate invertebrate species, you are doing something right. Conversely, if after a month your pond is as free of life as tap water, you have a serious water quality problem.

    Aquatic invertebrates — especially the shredders and chewers — initiate the breakdown of dead plant matter in the pond. They create the openings for bacteria and fungi to complete the decomposition process. In other words, they are essential for natural recycling of organic material in the pond.

    Moreover, many herbivores among the zooplankton specialize on eating algae. They can significantly reduce the green water “problem” without reducing the algae count to the point where the ammonia levels surge, threatening the fish.

    Yes, nature does try to keep things in balance. The trouble is that we tend to tinker with single bits of pond ecology without realizing they are not really isolated.

    For example, many hobbyists rely on pesticides, parisiticides and anti-microbials to treat their ponds. Putting the marginal effectiveness of these products aside, they will also kill beneficial pond bacteria, fungi and invertebrates. The irony is that they are not really successful in dealing with the target bacteria or parasite, but they will eliminate those species that add to pond water quality.

    Water quality means many things to many people. To most pond hobbyists good water quality means clear and clean water — water that allows their guests to appreciate the fish. In other words, most pond hobbyists emphasize the aesthetic aspects of water quality.

    But, to your fish, good water quality represents something quite different. It means environmental conditions that allow them to thrive. Aesthetics are irrelevant to fish. Focus on the water quality characteristics that make the pond a living system first. If you do this, the more ornamental characteristics will almost certainly fall into place.

  • Article Categories:
    Fish · Ponds and Koi