Battling Algae
by Ellen Feld, author of equine articles
and the "Morgan Horse" kids book series available at

It’s green, slimy, ugly, and hard to kill. Algae seems to thrive in almost any wet environment and once you scrub it away, it reappears days later. What are the best ways to control algae growth in your water buckets, troughs and ponds?

To understand how best to control algae, it will help to know exactly what species you are battling. Explains Mr. Willie Woode, Senior Conservation Specialist for the Northern Virginia Soil and Water Conservation District, “For water buckets and troughs, the container has water that is fairly temporary so you would be limited in the types of algae you find. Typically, you’d encounter two types of algae. The first is filamentous algae, which are microscopic units of cells joined to form strings, which in mass appears as a slimy mat of algae or stringy strands. The other is unicellular [also referred to as planktonic]. Although you can’t see it with the naked eye, it makes your water look green.”

What makes algae thrive in your water? “The key conditions,” continues Woode, “are nutrients, warmth and moisture. Warmth can be provided by the heat of the sun which is why algae in troughs out in a pasture grows faster than in a bucket in the barn. Nutrients can come from horse droppings in the water as well as hay, feed or leaves left in the water. Horses on pasture will also frequently come to a water trough with pieces of grass in their mouths. Nutrients from these plant parts are then released into the water. Also, if your water comes from a spring, or pumped from a pond or any other natural setting, it is more likely to have algae growth than water that is coming from a city water system. The pond water usually has nutrients in it to feed the algae whereas the city water will take a little longer time to accumulate enough nutrients to support significant algae growth.

“Another ideal condition that buckets and troughs offer is that the water is stagnant. Algae grows better in stagnant than fast moving water. So if you have a pond, consider installing an aerator in the form of a fountain or bubbler to reduce chances of an algae bloom. Occurrence of algae problems also depends on the management style of upstream properties. Over fertilized lawns or pastures are major sources of pollutants that support algae in ponds.”


Why worry about algae? Because, explains Woode, it can harm your horses. When algae first begins to grow, it isn’t a problem. Horses can drink the water without any adverse effects. But all algae goes through a cycle, first of growing, and then dieing and decomposing. Usually, the die-off will occur two to three weeks after the first appearance of green. As the algae decomposes, it gives off harmful toxins that if ingested, may cause a horse to colic. There is an odor the algae gives off when it is decomposing, but it is too subtle for humans to notice. Horses, however, will detect it and may sometimes hesitate or refuse to drink such water.

Woode insists that if you have algae growing in your buckets and troughs, then your watering system is not good enough for horses. “Good management,” notes Woode, “requires that you keep your water as clean as possible and change it every two to three days. If you change it that frequently, then there is little chance of algae establishing itself. You need to empty the bucket or trough and scrub it to remove any budding growth. However,” warns Woode, “if you just top off the container, you will have problems with algae because the remaining water may contain cells, younger growth or fragments of algae that will readily establish a new colony. People who use large tubs frequently just top them off because it is time-consuming and messy to empty. No matter what the containment is, it is imperative that you dump the water out, scrub, and re-fill with fresh water. Again, you need to do this every few days for buckets and at least once a week for large troughs.”

What if you still can’t get rid of the algae? Many people report success by adding apple cider vinegar to the water. “It works by dropping the pH,” explains Keith Diedrick, extension educator with the Ohio State University Extension Office.

“But it can get expensive. Also, although plenty of companies advertise it, there doesn’t seem to be a lot of scientific evidence to support its use. Catfish or goldfish are often added to troughs with minimal to virtually non-existent degrees of success; I do not see much scientific literature from non-biased sources that promote this practice as a viable solution. Note that the fish will add ammonia to a closed system. Algae is consumed, and waste is generated; that does not seem like a great solution to the problem.”

Instead, Diedrick suggests cleaning your buckets and troughs with chlorine. Ohio State University research shows that household bleach (unscented is best) at 2 to 3 fluid ounces per 50 gallons of water every 7 days is effective in killing algae. Copper sulfate is another option. “The concentration recommended,” notes Diedrick, “is 1 part per million (ppm), or 1.5 teaspoons of copper sulfate crystals dissolved in 4.5 oz of warm water (accelerates dissolving rate of the crystals) to treat 1,000 gallons of water in the trough or tank. This is a linear relationship, and if the tank size differs, nothing in the equation has to change, i.e., 500 gallons of water takes 0.75 teaspoon of crystals dissolved to treat. This treatment can be applied every 2-4 weeks as needed.”

Many farms use ponds to water their pastured horses. How do you control algae in such a large span of water? First, be sure that there is no runoff from your manure pile that is leaching into the pond. Manure will provide much needed nutrients for the algae. Keep your manure in a protected structure, build a birm around it to keep rainwater from leaching its nutrients and keep it far away from the pond.

Most chemical treatments are copper based and sold under different trade names. Unless you are proficient in chemical applications, Woode advises to use products with copper complexes rather than copper ions as the active ingredients. “Copper sulfates [copper ions] in the concentrations needed to kill algae will also kill your fish and all lower level organisms. To be on the safe side, when you use any of these solutions, it is best to treat a portion of your pond at a time instead of the whole pond. That way, if the chemical concentration becomes a threat to your fish population, they can migrate to other sections of the pond and survive. In contrast, when you use a copper complex it won’t harm the fish because it is a slow release chemical and not as harsh on organisms. But it is just as effective at killing the algae. Your extension agent should have a manual to identify the different types of algae. Once the algae is identified, your agent can give you a list of appropriate algaecides and rate them for you, from excellent to good to fair. If any chemical control is used, it is very important to determine how long such water bodies should be kept without being used as watering sources for your horses.”

For those who prefer a non-chemical solution, there are several options. The first is the use of grass carp. These large (up to 50 pounds), long-lived herbivorous from the minnow family are vegetarians with enormous appetites; and they love algae. Be aware that because these are non-native fish, a special permit may be required before releasing them in your pond. To prevent spread of the carp to other water supplies, there is a sterile triploid form of grass carp now available.

Another option is the use of barley straw, which has been popular in Europe since the early 90’s and is now being used in the states. Many pond management companies sell it in rolls and when added to the bottom of the pond, it releases a chemical that impacts the pH of the water and reduces the potential for new algae growth (it doesn’t seem to be useful for existing algae). The use of barley straw is not believed to be harmful to horses but check with your veterinarian.

Controlling algae is a constant struggle. But with good management practices, it can be controlled and even eliminated.

About the author: Ellen Feld has written for equine publications, both regional and national in scope, for over 20 years. In addition, she is the author of the multiple award-winning ‘Morgan Horse’ series. These books have won awards both from reading organizations as well as parent groups for their realistic portrayal of horses wrapped in fun, wholesome adventures that kids love. Learn more at


Ask us your horse questions! Read other horse FAQs

Share This Page!

Free Horse Newsletter
No spam, totally free! Fun horse articles, games, & news.

About / Contact
- Submit Your Writing - Advertise

© COPYRIGHT Ultimate Horse Sites Inc. 2000-2005
Content is copyright and not to be taken, copied, or used in any way without written permission.

Want to use our content? Write for permission please:

Use of the terms "Ultimate Horse Site", "The Ultimate Horse Site", "Ultimate Horse", "UltimateHorse", "The Ultimate Horse" have been in use since 2000 and use of variations of our name for any reason is prohibited.


The ultimate source for everything horse-- from informative articles on training and horse care, to horse games, names, and jokes! Free horse games