You’ve put a lot of time and money into building your backyard pond. Now, you just want to put up your feet and watch the koi flit around their crystal-clear water.
But what if the water isn’t clear? What do you do if your fish are hidden behind a haze of green-water algae, or your rocks are covered in green slimy gunk?
Disruptions to your pond’s ecosystem can tint your beautiful water an unsightly green or brown. The good news is you can keep your water clear relatively easily – without using harsh chemicals.
At a Glance: How to Keep Pond Water Clear
- Understand that a little bit of algae or discoloration is normal
- Use beneficial bacteria to starve single-cell algae that turns water green
- Add a wide variety of aquatic plants to starve string algae
- Add a larger biofilter
- Don’t overfeed your fish
- Don’t overcrowd your fish
- Don’t power wash your rocks
- Keep your water oxygenated
- Watch for any debris that may be washing into your pond
1. Understand the Most Common Causes of Water Clarity Issues
Your pond isn’t just a relaxing place to hang out. It’s a living, breathing ecosystem full of fish, plants, bacteria and other life that you can’t even see.
Lots of imbalances in your pond’s biology can cause different kinds of water clarity issues. If you want crystal-clear water, your first step is to determine what kind of problem you have.
Green Water (Single-Cell) Algae
Single-cell algae are tiny organisms that live in your pond and turn the water pea soup green. You’ll know you have a single-cell algae problem if your hand looks green or disappears entirely when you submerge it in the water.
Algae is not inherently bad; it’s just nature’s way of dealing with excess nutrients. Single-cell algae arrive in your pond as tiny airborne spores. These spores feed on nitrites, a natural byproduct of beneficial bacteria breaking down harmful ammonia in the water.
We can eliminate green water by introducing more beneficial bacteria into the pond. We’ll go into more detail below on how and why.
Green water algae will not hurt your fish. Pond owners outside the U.S. sometimes even embrace the natural beauty of an emerald-colored pond. (Most Americans, though, prefer to be able to see their fish!)
String algae is the most common type of algae in backyard fish ponds and waterfalls. It’s slimy and green, and you might even be able to pick it up with your hands. Unlike single-cell algae, string algae will not turn you water green. Instead, it clings to rocks, liner and plants, or floats on the surface of the water.
String algae is a natural, normal, expected and even beneficial part of your pond’s ecosystem. You will always have a little bit of it hanging around, but we can take steps to prevent it from running amuck.
Like single-cell algae, string algae arrives in your pond via airborne spores and feeds on excess nutrients in the water – in this case, nitrates. Check out our tips below to learn how we can cut off this food source and keep string algae at a minimum.
Debris, Tannins, etc.
Algae is usually green. If you have brown water, the problem is probably some other type of organic material in the pond.
Leaves, mulch and other natural sources contain tannins, which are organic color compounds that can make your water brown. If tannins are the issue, the water will usually be mostly clear with a slight brownish tint – kind of like a cup of tea. If your pond is brown and cloudy, on the other hand, something has probably stirred up sludge on the bottom of the pond.
We can fix both these issues fairly easily with flocculants, carbon or other tools. Keep reading to learn more.
2. Add Plenty of Beneficial Bacteria
Green water is caused by single-cell algae, which feed on excess nitrites in the water. Beneficial bacteria also consume nitrites. The secret to clear water, then, is to grow enough bacteria that all of the nitrites are consumed and none is left for single-cell algae – starving the algae so it doesn’t grow and make your water look green.
But isn’t bacteria bad?
No, not in this case. Beneficial bacteria is a product sold specifically for pond maintenance and is one of the safest and most natural helpers you can add to your pond water. You don’t have to worry about adding too much – unlike with algaecides, an overdose won’t kill your fish or plants – and it’s safe for pets and humans.
You can either add bacteria manually or buy an autodoser that will do it for you. Exact amounts will depend on the size of your pond and the specific product you choose (liquid, tablets. etc.). Just follow the directions on the back of the container.
2. Increase Filtration
Your pond should have two types of filtration: biological and physical.
Physical filtration removes leaves, fish waste and other debris from the pond before it has a chance to cause problems. The best type of physical filtration is a built-in pond skimmer. Similar to a swimming pool skimmer, pond skimmers suck debris into a basket that you can easily empty as needed. Built-in skimmers are ideal for rubber-lined ponds, but you can also purchase floating skimmers for small ponds.
If your pond currently has no filtration at all, installing a skimmer is the first step you should take to keep your water healthy. A skimmer prevents debris from building up on the bottom of the pond, where it can turn into sludge and contribute to harmful ammonia build-up. (Can’t add a skimmer right now? Then it’s time to invest in a good skimming net so you can remove debris by hand.)
Biofilters are a kind of party spot for those algae-starving bacteria. And anything that makes your bacteria happy will make your pond happy.
Biofilters/BioFalls are essentially tanks filled with nooks and crannies – in the form of BioBalls, lava rock or anything else with lots of surface area – where bacteria can reproduce. Having a large biofilter will help those all-important bacteria to thrive.
Biofilters can take lots of forms. The most common is a BioFalls: a durable plastic container that holds filter media and forms the beginning of a waterfall. Water flows into the bottom of the falls, swirls around the bacteria-rich interior, then spills out the top. These types of filters are easy to camouflage with plants and rocks.
Large ponds, meanwhile, might have constructed wetland or bog filters. Or if you have a tiny pond, you might opt for a decorative urn filter or small pond waterfall filter. No matter what kind of biofilter you choose, we recommend going as big as your space and budget allow. More biofiltration means more beneficial bacteria, which means happier fish and clearer water.
We also recommend building your pond with gravel on the bottom. Gravel, in addition to looking nice and adding traction on top of slippery liner, gives bacteria even more nooks and crannies where it can grow.
One final note on biological filtration: You ideally want your biofilter to be separate from your physical filter. While some filters claim to offer both kinds of filtration in a single unit, it’s better to have two filters so you can clean debris from the physical filter as often as needed without disturbing the bacteria in the biofilter.
Pond Kits with Included Filters
If you haven’t built your pond yet – or are planning a full renovation – look into kits that come with a physical filter and a biological filter. Buying a kit will save you a significant amount of money, while also ensuring the size and type of filtration is appropriate for the pond you’re building.
4. Resist the Urge to Power Wash
You can give your pond a head start on algae control by physically removing string algae from your rocks. There’s a big difference, however, between pulling up a few handfuls of gunk and dragging out a pressure washer.
Washing your rocks will remove pieces of the ecosystem that your pond has spent so long building up. This ecosystem is so complicated that we can never replicate it with chemicals, so we never want to scrub or pressure wash to the point where we get rid of that ecosystem entirely. The algae will come right back anyway.
If you must give your rocks a bath, remove debris and gunk as gently as possible and rinse them with water from the pond instead of a hose. The same goes for your biofilter. Only clean it once per year – ideally in early spring – and be as gentle as possible.
5. Embrace Flower Power
When bacteria eat algae-enabling nitrites, they produce nitrates. These nitrates feed string algae: that stringy, fuzzy stuff that clings on to your rocks.
While some amount of string algae is normal for a healthy pond, you can keep it controlled by adding a wide variety of plants to your water. Different types of plants will eat different kinds of nitrates. An iris will suck up one kind, and a lily will consume another. A blue flower will chow down on one, and a pink flower yet another. A plant that blooms in May will eat a different kind than one that blooms in September.
These plants will work together to prevent algae from growing in the first place, so you don’t have to worry about killing it later.
5. Cut Back on Fish Food
Your fish won’t get fat if you overfeed them, but excess food will harm your pond.
Only feed your fish as much as they’ll eat within about 5 minutes of you throwing it in the pond, and don’t feed them more than once or twice a day. Excess food could end up in the bottom of your pond, where it will rot and contribute to sludge and ammonia build-up.
You never need to worry about your fish going hungry; they will snack on algae, mosquito larvae and other materials in the pond. You can also supplement their diet with chunks of watermelon, orange slices or Koi Krunchies to help them pack in nutrients.
Cut back on feedings in mid fall, and stop feeding fish entirely when the water temperature drops below 55 degrees.
6. Don't Add Too Many Fish
The best way to keep pond water clear is to minimize the amount of organic gunk in the pond. And a lot of that organic gunk comes from fish waste. Too many fish means too much fish waste, which means dirty, unhealthy water.
So how many fish should you have? The answer is a little more complicated than a simple number of fish per gallon. It all comes down to how well your pond ecosystem handles the fish waste. The bigger the biofilter, the more fish waste you can process.
Your pond will let you know if you have too many fish. Some common signs of overcrowding include
- Difficulty keeping the water clear, despite regular doses of beneficial bacteria
- Excessive string algae, despite lots of plants in the pond
- High ammonia levels
- Fish illness/death
If you find yourself with too many fish, it’s time to improve your filtration system (i.e. install a larger biofilter and/or skimmer), or find new homes for your fish.
9. Watch Your Pond's Surroundings
Is your water cloudy or brown? Your problem may have an easy solution. Look for external debris that might be washing into your pond, like fresh mulch, pollen, tree sap or soil.
Cloudy water is common right after a pond is built, after a heavy rain, after adding plants or any other time something has stirred up debris. This cloudiness will usually clear on its own. If you want to speed it along, add a flocculant like Rapid Clear, and/or place an extra-fine filter mat in your skimmer.
Clear, tea-colored water, on the other hand, happens when tannins dye the water. These organic compounds usually come from leaves and other tree debris, and they won’t hurt your fish.
Tannin-related discoloration is most common in the fall, or you might see it year-round if the pond is under a large tree. You can get rid of the brown tint by adding activated carbon to the water, or you can choose to embrace the natural color.
10. Think Carefully Before Adding Algaecide
Algaecides remain one of the most popular options for ridding ponds of green gunk. They do provide quick results – but be careful.
Algaecides do not actually treat the main cause of your water problems, just the symptom. Contact algaecide powders, for example, are usually safe to use for spot-treating string algae, but they won’t do anything to remove the excess nutrients from the pond (and could even increase the nutrient load if you don’t remove dead algae from the water).
Avoid liquid algaecides if your pond contains fish. These products rapidly deplete the water of oxygen, leading to fish kills when dosed incorrectly. (If you must use liquid algaecide, err on the side of under-dosing, and make sure the pond has plenty of aeration. Do not use liquid algaecide on hot days.)
Lots of other tools on the market aren’t necessarily dangerous, but they are usually unnecessary. These tools include UV lights, IonGen filters and similar products. We don’t use them in our pond builds because we simply don’t need them. In our 30+ years of experience, we’ve found that the natural methods are usually the best methods.