How to Treat String Algae in a New Zealand Pond
String algae is a filamentous algae that grows on rocks, waterfalls and pond surfaces. It thrives when there are excess nutrients and sunlight, and is most common in new ponds before the ecosystem is fully established. In most cases it can be managed through manual removal, consistent beneficial bacteria treatment, and improving biological balance - without harsh chemicals.
String algae is one of the most common things pond owners contact us about. Those green, hair-like strands clinging to your rocks, trailing through your waterfall, or forming mats across the surface - it can look alarming, especially if your feature was crystal clear when it was first built.
The good news is that string algae is almost always manageable, and understanding what's causing it makes all the difference. This post covers both ecosystem ponds and pondless waterfalls, and what you can do about it in New Zealand conditions.
First - Is String Algae Actually a Problem?
Not always. A small amount of string algae is a completely normal part of a healthy, living water feature. It's one of the first signs that your pond or waterfall is ecologically active - it absorbs excess nutrients from the water and provides some oxygenation and habitat for micro-organisms.
Aquascape, the ecosystem technology we build every Southern Waterscapes feature on, describes string algae as the weeds in a new garden: they dominate early on, before the ecosystem is fully established, and naturally reduce over time as plants, bacteria, and biological balance take hold.
So if you've got a newer feature and you're seeing string algae — that's expected. It doesn't mean something has gone wrong.
String algae becomes a problem when:
It forms thick mats that clog your waterfall or filters
It covers rocks and significantly changes the appearance of the feature
It starts decomposing in the water, reducing oxygen and stressing fish
That's when it's time to act.
Why Is It There? The Main Causes
String algae thrives when there are excess nutrients in the water and plenty of sunlight. In a healthy, balanced ecosystem pond or pondless waterfall, the plants and beneficial bacteria compete with algae for those nutrients and keep it in check. When the balance tips, algae wins.
Common triggers in New Zealand conditions:
Spring growth flush - the most common one. As water temperatures rise in Canterbury spring, algae responds fast. Your biological filtration and bacteria haven't fully reactivated yet after winter, so algae gets a head start. This usually corrects itself as the season progresses.
New features - a freshly built pond or pondless waterfall doesn't have an established colony of beneficial bacteria yet. Algae fills the gap while the ecosystem matures. Typically most noticeable in the first season.
Too much sunlight - a feature with little shade and full Canterbury summer sun will produce more algae than one with some coverage. Aquatic plants like water lilies help here, both by shading the water and by competing directly for nutrients.
Leaf and organic debris - Canterbury's deciduous trees shed heavily in autumn, and leaf matter breaking down in the water is a direct nutrient source for algae. Keeping debris cleared from around the feature makes a real difference.
Overfeeding goldfish - uneaten food breaks down and adds nutrients. Feed only what your fish will consume in a few minutes.
Underperforming bacteria - beneficial bacteria is the engine of a healthy pond ecosystem. Without enough of it working consistently, nutrients build up and algae capitalises.
What to Do About It - Ecosystem Ponds
Step 1: Remove it manually first
The quickest way to reduce string algae is to physically remove it. Use a pond net, brush, or your hands to pull out large clumps. Getting it out of the water lowers the nutrient load immediately and gives your treatments a better chance of working.
Remove it carefully - disturbing string algae can release spores into the water. Once removed, compost it away from the pond.
Step 2: Treat with the right product
Aquascape's water treatment range is built around multi-formulation products - each bottle contains a powerful blend of ingredients working together, so you're not having to source and add separate products for each job. You switch between products depending on what your pond needs, rather than using them all at once.
For string algae and water clarity issues, the product to reach for is:
Aquascape Clear for Ponds - formulated specifically to address algae and restore water clarity. Contains a blend of beneficial bacteria, phosphate binder, flocculant, and pond detoxifier - multiple bacteria strains working together to address the root cause rather than just the symptom. Fish safe and plant safe.
Once clarity is restored, switch back to Aquascape Maintain for Ponds as your ongoing treatment. Maintain contains the same powerful multi-formulation blend to support healthy biological balance and keep water conditions stable - it's your baseline product for keeping the pond performing well week to week.
If you have an Aquascape auto-dosing system:
Use the SL versions — Clear for Ponds SL and Maintain for Ponds SL. These are formulated for the dosing system and deliver the right amount of treatment automatically every day, so the bacteria and active ingredients are working consistently without you needing to remember.
If you don't have an auto-dosing system:
We stock Clear for Ponds and Maintain for Ponds in standard 473ml pump bottles, designed for existing pond owners to dose manually. These are available directly from our online store.
Step 3: Address the underlying cause
Treatments help, but if the conditions that caused the algae aren't addressed, it'll keep coming back. Think about:
Adding more aquatic plants — water lilies, marginal plants, and submerged oxygenating plants all compete with algae for nutrients. The more plant coverage you have, the less algae will thrive.
Clearing autumn debris promptly — don't let leaf matter sit in or around the pond
Checking your filtration — is the biological filter running properly? Has it been cleaned recently in a way that may have disrupted the bacteria colony?
What to Do About It - Pondless Waterfalls
String algae in a pondless waterfall typically appears on the rocks and spillway rather than floating freely, because there's no standing water body for it to colonise.
The treatment approach is simpler than for ponds - and because all Southern Waterscapes pondless waterfall builds include an Aquascape auto-dosing system as standard, the SL treatment range is what applies here.
Manual removal first
Same as for ponds - physically remove what you can from the rocks. A stiff brush works well for dislodging algae from stonework.
Use the right product for your feature and use it consistently
There are two SL treatments for pondless systems - which one you use depends on your feature:
Aquascape Clean for Fountains & Waterfalls SL - for pondless features that have fish or aquatic plants. Fish safe and plant safe.
Aquascape Prevent for Fountains & Waterfalls SL - for pondless features without fish or plants.
Both work through the auto-dosing system, delivering treatment daily without any effort on your part. Consistency is what keeps algae in check - the dosing system handles that for you.
The Auto-Dosing System: The Easiest Way to Stay on Top of It
The most consistent results we see from clients are from features running an Aquascape auto-dosing system. It delivers the right amount of treatment product into the water daily, automatically - so the beneficial bacteria and water treatments are working constantly, not just when you remember to add them.
For ecosystem ponds, pair the system with Maintain for Ponds SL. For pondless waterfalls, Prevent or Clean for Fountains & Waterfalls SL is the right choice. The dosing system works with all of these - just load the bottle that suits your feature and let it run.
The Aquascape Automatic Dosing System is available through the Southern Waterscapes online store and comes standard on all new builds we complete.
When to Call Us
If your feature has significant string algae that isn't responding to treatment, or if you're unsure whether your filtration is performing properly, it's worth getting us in for an assessment. Sometimes what looks like an algae problem is actually a symptom of undersized filtration, a bacteria colony that needs resetting, or a debris issue in the reservoir.
Our annual clean-out service is also a good reset point - clearing accumulated debris from the system in spring gives your beneficial bacteria a clean slate to work from, and often dramatically reduces algae through the season.
Get in touch if you'd like to discuss your feature or book a clean-out.
Southern Waterscapes Ltd is New Zealand's only Certified Aquascape Contractor, based in Rolleston, Canterbury. Water treatment products are available from our online store.