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5 Ways to Control String Algae in a Koi Pond

Controlling String Algae in Koi Ponds San Diego, CA

If you own a koi pond, you’re almost certainly familiar with the nuisance known as string algae. Though it’s at its worst in the spring when plants aren’t quite growing yet but algae sure is, it also becomes a nuisance in the summer months when the temperature gets hot. It can even rear its ugly head year-round, even through the cold of winter. The problem it poses is mostly an aesthetic one—after all, it mats together and produces unsightly clumps in your pond—but if left ignored it can even negatively impact the amount of oxygen in the water itself, potentially harming your fish. Luckily, there are a few ways you can start reducing the presence of string algae in your outdoor water feature. Follow these tips, provided by the knowledgeable staff at Aquatic Warehouse.

1.  Remove It Manually

This step is unavoidable. No matter what else you opt to do, first you’re going to need to get in there and physically clear out as much string algae as you can. It doesn’t have to be with your hands—a rake or a brush will do just fine—but you’ll want to make sure this is step one of your pond-clearing regimen.

2. Enlist a Little Help

You don’t have to tackle this alone. Other plants (and bacteria) can be your allies. Some carefully selected flora—especially plants that grow quickly—can choke out string algae. The introduction of barley, in bales or liquid form, can also discourage new string algae from growing by releasing hydrogen peroxide. Furthermore, bacteria can be of use as well. Available in liquid or granular form, certain types of bacteria can eat away at the sludge dead algae leaves behind. One of the best organic options is ClarityMax powder additive:


This product works great at reducing the nutrients that cause algae and is excellent at clarifying the water as well.

3. Use Chemicals

When carefully applied, algaecide can help you quickly combat unwanted string algae growth. You’ll want to be selective about the brand you use and where you pour it (if you have a high-flow feature such as a waterfall installed, it’ll work best if applied there). This isn’t the best application for animals living downstream though.

4. Get to the Root of the Problem

The most common causes of unmanageable amounts of algae are high phosphorous and pH levels. If you suspect your koi pond might be suffering as a result of either of these, it’s best to test your water. You can buy a kit to do it yourself. Once you’ve determined which of these is plaguing your pond, there are quick steps you can take toward reducing either or both. If your pH is too high, there are correctors on the market that can help you adjust it accordingly. You may also want to check if you have rocks (such as limestone) that are exacerbating the issue or if you have too many fish in the same pond. If phosphorous is the problem, you can also purchase phosphorous removers. Alternatively, make sure to check that no fertilizer from your lawn is leaking into the water (or start using a non-phosphorous alternative.)

Also, just like an aquarium, do water changes! A lot of people just add water to their ponds and think they’re doing water changes. We wish it worked that way. Instead, take out some water twice a month and water your vegetable garden with it. Fish emulsion is very expensive fertilizer, and you have tons of it available for free in your own backyard.

5. Don’t Overfeed Your Fish

It’s also entirely possible (and probable) you’re giving your fish too much food and that the string algae is feasting on it. If you’ve noticed food lingers on the surface of the pond well after feeding time is over, it’s excessive. Algae is more than happy to thrive on the leftover nutrients.

At Aquatic Warehouse, we carry everything you need to keep your koi pond healthy and beautiful, from filtration to pond vacuums. If you have any questions about caring for your pond, don’t hesitate to give us a call at 858-467-9297.

Contact Information

  • Address: 5466 Complex Street Suite 204
    San Diego, CA 92123
  • Phone: (858) 467-9297
  • Email: sales@aquaticwarehouse.com
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