Dealing with Green Water, Mossy Algae and String Algae

Dealing with Green Water, Mossy Algae and String Algae
The most common pond care question is how to control algae in the Practical Garden Pond. Here we will identify algae in three categories: green water algae, mossy algae and string algae. All three must be addressed in order to keep your pond algae-free.
  1. Green water (Planktonic algae) is composed of millions of microscopic free-floating algae cells that reproduce quickly and eventually color the water a deep shade of green. 
  2. Mossy algae is a light coating of soft, moss-like algae covering some of the underwater surfaces of the pond. 
  3. String algae (Filamentous algae) can amount to massive growths of yucky, stringy, smelly algae that will obscure your view and gum up your pond and equipment.

For the record, none of these algae forms actually harm the fish in any way, and each of the three kinds of algae should be handled differently. I will briefly talk about green water and mossy algae, but I will spend most of this article dealing with string algae – the biggest pond algae problem.

Green Water Algae: While all these algae problems originate from a combination of sunlight and dissolved nutrients in the pond, green water algae can be eliminated without addressing the root cause. The bad news is that the micro particles end up everywhere making it impossible to contain green water. The good news is that, since they are free floating in the pond and small enough to go through your filter media and pump, they will be exposed to a UV light if you build one into the system. Therefore, with a little investment in a properly sized UV light, green water can be easily controlled without becoming a botanist!

To completely control green water with a UV, you need a pump that turns your pond over at least once and preferably twice per hour. For example, in a 1000 gallon pond we recommend a 2000gph flow rate (less is needed in ponds over 10,000 gallons). You need to buy a UV in the wattage that the manufacturer recommends for your flow rate and total gallons of pond water.

For example, a 40 Watt Aqua Ultraviolet in line UV unit will eliminate green water in ponds up to 6000 gallons and in flow rates up to 3000gph. If you have a 2000 gallon pond, by pond size you would need only a 15 watt Aqua UV unit, but if your pump is pushing 3000gph over your water falls, you may still need a 40 Watt UV, since the flow rate in which a 15 Watt unit will effectively eliminate green water is only about 1800gph maximum. Across the board, the textbook recommendation is one Watt of UV for every 90 gallons of water flow through the UV unit.

Mossy Algae: The practical approach to a thin layer of mossy green algae on the rocks or liner at the bottom of your pond is very simple: learn to appreciate it. Seriously, a thin coating of green is natural for a pond, and it actually improves the appearance of your pond if you have a liner bottom. If you have beautiful stones, I can see why you might prefer a pristine look, but it makes a lot more sense to recognize the value to fish and other pond-dwelling creatures of a little bit of natural green algae and just learn to appreciate it.

String Algae: The “learn to appreciate it” advice does not apply to thick growing gobs of smelly algae obscuring your fish and clumping up in your filtration. So the question remains, concerning string algae – what do you do?

The bad news is there is no perfect solution. The recent improvement of ionizers for pond use line has given a possible solution to those who want it.

But ionizers work by putting small amounts of zinc and copper into the water, and if you don’t monitor the copper content correctly, you run the risk of killing your fish. For this reason, Practical Garden Ponds recommends ionizers for pond-less waterfalls, fountains and ponds with no fish in them, where it will do a great job risk-free. But while ionizers can be used carefully with good results even in fish ponds, we tend to look for other solutions.

It is important to realize that there are nitrate, phosphates and other dissolved nutrients in the water that are byproducts of fish waste and decaying organic debris. These, along with heat and abundant sunlight, are the primary contributors to excessive algae in your otherwise beautiful pond. Understanding this can help you get a handle on controlling algae blooms.

Algae control is not one action, but the maintaining of proper balance in the pond. Here are the components that contribute to a higher water quality standard and long-term algae control:

  1. Preventing nutrient buildup: The first line of defense is always prevention. It is better to eliminate the cause of water quality problems than to try to get rid of their symptoms.
    • Population: The more fish you have, the more waste will be produced in the pond. Choosing to be satisfied with a healthy modest fish population is a first step in reducing algae problems. The more fish you have the more difficult controlling algae will be.
    • Feeding: It’s not just about how many fish you have. Over-feeding your fish is a very common cause of many kinds of water quality problems. Feed your fish moderately to encourage them to eat algae and other natural food sources in the pond. If you are feeding your fish more than once or twice a day, or more than they eat completely in just a couple of minutes each time, you are overfeeding. Also, consider a feeding ring (Shown to the left) to keep the fish food from floating into your skimmer and adding to the organic waste in the pond. Note: Large Koi (over 8”) will eat a significant amount of string algae if they are not overfed.
    • Debris: Remember that there are other sources of organic waste besides your fish and fish food. Control leaves and debris in your pond with a proper skimmer to catch and remove floating debris before it saturates and sinks to the bottom. Consider pond netting in the Fall, to keep out excessive debris. You may even want to invest in a pond vacuum for easily removing settled debris from the bottom of the pond (Ask Nate at Practical Garden Ponds for a pond vacuum value comparison). In addition to removing debris from the bottom of the pond, vacuuming also provides a partial water change, removing water from the bottom of the pond where it is most saturated with minerals and organic waste. Partial water changes give you the opportunity to re-introduce pure clean water in its place, like a breath of fresh air for your pond.
  2. Competition for nutrients: Depending on the kind of pond you have, you may want to introduce water plants to the pond. A good array of water plants helps shade the pond and competes for the dissolved nutrition in the pond. Be aware, however, that large fish will constantly pick apart the plants and dig in the pots. Unhealthy plants, floating debris and uprooted organic material simply add to the problem of water quality and feed your algae problem. If you can maintain healthy plants in your pond, they are an asset– especially large shade producing plants like water lilies and fast growing underwater plants like anacharis (elodea). But extra organic debris is a liability. The best way to integrate healthy plants into a pond with large fish is in a natural bog where the plants are protected from the fish and the nutrient rich water is actively channeled past the roots of the plants (see Bog Filtration below in Biological Filtration).
  3. Removal of nutrients and organic debris from the system: Your final defense against debris and excess nutrition in the pond is a properly designed, well maintained and properly functioning filtration system.
    • Adequate filtration: I am amazed how many otherwise incredible ponds lack adequate filtration. There are three basic kinds of filtration: particle filtration, chemical filtration, and biological filtration.
      • Particle filtration is essential. It is very important that you trap and remove large organic matter before it breaks down and becomes an invisible threat to your pond. This can be done with a good skimmer, a leaf basket and filter pad, a bead filter or other sediment-trapping filters that offer an easy way to remove the trapped sediment. Filters should be backwashed or otherwise relieved of the trapped debris frequently so that the trapped debris does not continue leaching nutrients back into the system.
      • Chemical filtration is usually considered impractical in a large pond. Phosphate removers and treated carbon are certainly assets, but require frequent replacement which is expensive and therefore generally impractical. These are available and can be used to effectively remove nutrients and “catch up” in a pond that is out of balance.
      • Biological filtration is the lifeblood of your pond’s filtration system. You must have adequate colonies of beneficial bacteria to handle the amount of water and dissolved waste in your pond. This can be accomplished through a quality filter, home-made media beds like lava rock or skippy filters, or through the addition of a beautiful natural bog. A bog is an area in which the water is dispersed through the bottom in an open cavity or through a maze of perforated pipe, up through a bed of gravel (pea-sized to ¾”). Plants are placed directly in the gravel (no dirt!) so that the nutrient rich water actively flows through their root bed. An alternative is an under-gravel filter bed in the bottom of the pond. The addition of aerated biological filtration like the Helix Moving Bed Waterfall filter will dramatically increase your biological filtration and improve your water quality.
      • Whichever form of filtration you choose, it needs to be adequately sized for your pond and easily maintained. Filters will provide specifications that will tell you how much pond water they can handle and at what flow rate. However, some manufacturers overrate their product. In general, filters that are very small in physical capacity are not adequate for medium or larger ponds. Common sense tells us that large colonies of bacteria require large amounts of surface on which to cling. From my experience, I have learned to trust the Aqua Ultraviolet Ultima II as a filter with overkill. You can trust their stated capacities, but they are also larger and more expensive than many other filters that claim similar results. I like the Savio F100 Waterfall filter as an inexpensive large biological filter falls. It is appropriately sized for ponds up to 5000 gallons. But again, this filter is the size of a 30-gallon garbage can– not the size of your kitchen waste basket. It also has a large amount of media inside of it. When building bogs, keep in mind that they should always take up about 10% of the pond. Adding aeration, like in the Helix Moving Bed and Bio Mechanical Reactor, dramatically increases the density of beneficial bacteria colonizing in your filter.
    • Aeration: Straight aeration directly in your pond is also an asset, growing beneficial bacteria, disturbing algae growth and increasing oxygenation and circulation in the pond. Oxygen is essential to healthy colonies of beneficial bacterial and it accelerates the nitrogen cycle. In addition, fish need dissolved oxygen in the water and can be robbed of oxygen when organic debris is breaking down in the pond. While moving water such as a waterfall adds oxygenation to the pond, additional aeration is always a good thing.
    • Circulation: A pond should always circulate completely, with no areas of the pond left stagnant. The entire pond volume should flush through the filtration system at least once every hour, and preferably twice or more. Filtration should run 24/7 during spring, summer and fall. Turning off your filtration to save electric is not an option. If electricity usage is a critical issue, ask us about air-driven filtration systems that can run on as little as 60 Watts.
    • Filtration maintenance: It is important to understand that just having good filtration is not enough. Good filtration must be maintained. Most filters do not actually remove debris from the pond automatically – they simply trap the debris inside the filter. It is still in the water flow, and organic debris can still leach nutrients into the water. With most filtration systems, it is up to the pond keeper to remove the debris from the filter through regular maintenance.
        • To prevent decay within the system, remove debris from leaf baskets and filters frequently. Rinse excess muck from filter pads at least monthly (weekly is recommended). The same is true for doing a backflush of pressurized filters like the Ultima. Trapping debris is only half the battle; you must also remove it. The Savio F100 Waterfall filter has a great feature – a built-in bottom drain. With the optional Bottom Drain Kit (tubing and a knife valve), the Savio F100 can be cleaned of sediment just by opening the bottom drain with the pump running. This is a great feature – but only if you open the drain periodically! The one filter on the market that actually removes debris from the water system is the Oase ScreenMatic. These incredible filters also include phosphate removing canisters, making them among the best filters on the market for preventing and fighting an algae problem.
        • Because it is difficult to physically remove debris from the bottom of a bog, it is very important that a regular maintenance routine for all natural bogs includes thinning and pruning the plants, as well as regularly using a special sludge formulation of beneficial bacterial like Microbe-Lift Sludge Remover.
        • No matter what form of biological filtration you use, when you radically clean out the debris, you are also likely to diminish the beneficial bacteria population. You can minimize any negative effect by gently rinsing the sediment out with pond-temperature water, but it is still very important to replenish the healthy targeted beneficial bacterial regularly with a trustworthy source like Microbe-Lift PL. Biological remedies are natural and take time to work, so give them time. Freeze-dried beneficial bacteria are more stable and cost effective than liquid varieties. Consider Microbe-Lift Maximum Clarity Plus, Crystal Clear ClarityMax Plus, or a reputable similar product.
  4. Other factors to consider:
    • Shallower (and therefore warmer) ponds have more algae problems. At least 40% of your pond should be a “deep zone” of at least 2’ depth in a water garden (plants and small fish) and 3’ depth or more in a serious Koi pond.
    • Rainwater run-off should bypass the pond. Rainwater in the yard contains many impurities, possibly even lawn fertilizer and other organic debris and should not be allowed to pollute the pond. Rainwater runoff is a heavy contributor to algae blooms.
      • If reasonably practical to do so, maintain a pH between 6.8 and 7.6, with 7.0 being optimum. Algae will thrive in highly alkaline water. Under no circumstances should your pH be higher than 9.0 or lower than 6.5.
      • Spring cleaning is best done in moderation. You should remove excessive debris and sediment from the pond. It is not necessary or beneficial to sterilize the pond. If you have to clean your pond or filtration system completely, always reseed the beneficial bacteria.
      • Learn to expect a bit of an algae bloom each spring while your pond is waking up from the winter doldrums. Be proactive with early growing plants, beneficial bacteria and do everything you can to minimize debris.
  5. Water treatments: I hope I have made it clear that an ounce of prevention is worth a pound of cure. However, no matter what you do, severe weather conditions, growing fish populations, and seasonal issues may sometimes give algae a short term advantage. When needed, there are some good algae chemicals on the market (like AlgaeFix) that can be effective. Just remember that if you kill the algae you are also adding to the organic debris in your pond, so putting your money into water quality makes more sense in the long run than putting it into algaecides. Therefore, regular use of beneficial bacteria and barley straw extract is recommended. You can even use a proper salt content to be both medicinal to the fish and to retard algae growth. Use only pure (non-iodized) salt. It is also strongly recommended that you have a meter for monitoring salinity if you plan to maintain a salt level in your pond.

Conclusion: Most people are looking for a “quick fix” to algae problems, when in reality the best defense against algae is a good pond design and long-term regular maintenance of that healthy pond. Algae problems are most severe in two situations: a new pond that is not yet in good balance and an older pond where incremental lapses in maintenance have caught up over time. Water quality slowly slips as fish size and debris build up.

There are many factors contributing to algae growth, but all in all your best defense against algae is to limit your fish population, keep feeding and debris under control, maintain a good filtration system, and perform routine maintenance like cleaning the filter and vacuuming. All these actions work together to prevent algae from taking over the pond. Include regular injections of healthy beneficial bacteria, as it is far better to put your money into prevention than into algaecides.

With a little bit of long-term investment in these simple steps, you can spend less time dealing with algae and more time sitting by your pond, enjoying it with family and friends.

Nate at Practical Garden Ponds

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